Finished an insanity run with an infiltrator. The worst part about it was limited ammo restricting the usefulness of the sniper rifle, but that was more of a "less fun" issue than a "too hard" issue. As several people have pointed out, Insanity isn't really THAT bad except in a few certain places. These were my rough spots and how I handled them:
1. Mordin's recruitment mission. This mission would be a cakewalk if it wasn't, in most cases, the first one that you do. As a level 3-5 noob, though, this one is ROUGH on insanity because of the ungodly regeneration rates of the Vorcha and Krogan. Make sure that you bring Jacob and train him in at least level 1 of incendiary ammo. Learning to shoot the tanks on the flame troopers is a very beneficial trick for this mission. An interesting note about the Blood Pack is that they are probably the toughest merc band for you to fight in the early game because of the difficulty in taking down charging Krogan and the Vorcha's hyper regeneration rate. In the late game, I find them to be, by far, the easiest to handle. Take Grunt with squad incendiary ammo and Mordin with heavy incinerate and they become almost trivial.
2. Garrus' Recuritment Mission: Yeah... this one is a ****. I never had Garrus die on in any of the lower difficulties, but I think I had to reload this mission 5 or 6 times, and at least 3 of them were because Garrus died. I will say that is one mission that is very well suited for an Infiltrator. A good sniper perch, abundant ammo, and a great field of fire. Nobody ever got into Garrus' base, but when the Blood Pack storm into the base, things get dicey. That damned Krogan boss that you fight at the end is just nuts. I agree with Garrus about his freakish regeneration rate. Even after I shot off all his defenses, he was regening faster than I could hurt him with bullets. I wound up having to whip out the grenade launcher to finish him off... and it took 3 or 4 shots with it to do the trick.
3. Grunt's recruitment mission: This isn't a hard mission in and of itself, but one area was really hard for me. It's the spot about halfway through the mission where infinite Krogan warriors spawn and keep coming at you. It wasn't AWFUL, but I did have to reload a few times. The end battle on this level would have been pretty tough, but I figured out on my own that you can fight from the doorway until the Krogan stop coming (there are only 5). When they stop, the mech and the merc aren't that tough on their own.
4. HORIZON!!!: The final fight at the control tower is, in my opinion, the toughest fight in all of ME2 - if you don't cheat. I didn't have much trouble with the Praetorian because it's pretty easy to just kite him around the control building in the middle. The hard part is the first stage with the two Scions and the multiple waves of Husks. I didn't learn about the cheat until after I had passed the mission, so I did it the hard way. What I finally had to do was just storm like crazy for the long "porch" area to your left. One husk spawns there, but you can take him out with a quick power combo and immediately take cover. The first few times I did this, I tried having Grunt and Miranda take cover at each end of the porch (near the entrances) while I took the middle near the heavy weapon ammo box. Didn't work. No matter how quickly I got them into cover, they were dead in seconds from the Scion attacks... mostly because they just wouldn't use the cover effectively. I found, after several reloads, that it's better to just let their AI's control where they go and move... I have no idea why that is the case, but it is. Anyway... take cover in the middle, whip out your grenade launcher and focus it and all of your allies powers on the front Scion. Here's the trick... hold at least a little of your ammo until each incoming wave of husks moves past the Scion, THEN shoot it. If your aim and timing are good, you will hit the Scion, and the blast radius from the grenade will kill all of the husks too. The hard part is keeping the husks from rushing you and burning down one of the Scions before dying. Once you manage to kill one of the Scions, things get MUCH easier. You can actually move off of the porch and fight it from behind the crates. Finally managing to do this, however, took me almost 30 reloads. This was the most frustrating part of the game for me, buy far. If you don't mind being a bit cheesy, you can actually breeze through this encounter. Apparently, when you first open the gate to the area, you can actually see the two Scions in the back of the area. You can snipe them down from range without ever triggering the encounter.
It sounds almost ridiculous, but I honestly didn't have any more trouble with anything (no more multiple reloads in the same area) until I got to.
5. The Collector Ship. Yep... this one sucked hard. Two very tough spots. The part with the platforms was incredibly frustrating... mostly because it's just impossible to keep your squadmates alive. Once again, I had more luck just leaving them on auto pilot that I did in trying to have them take cover somewhere. No idea why the "take cover here" command is interpreted by the game as a death sentence, but that seems to be the case. Anyway, I finally found two tricks in this area. The first is to get proficient at dodging Harbingers hairy black ball thing that he throws at you - the thing that knocks you out of cover. Having that thing pop you up and stun you just as a Scion shockwave is coming through is BAD. The second trick comes after you kill all of the lackeys and are left with only the Scions. As soon as you kill one of them more bad guys will show up, so, attacking from range with either warp or incinerate, slowly work each of them down just a tiny sliver of armor. Then, very quickly take out both of them. You'll get more bad guys, but won't have any Scion artillery to worry about... which makes the fight waaay easier. The second rough spot, and the one that I was worrying about in advance (it gave me fits on Hardcore) is the Praetorian fight in the open cavern. I prepared for this by doing two things (sort of an experiment). I brought the Cain, and I got ALL of the heavy weapon upgrades before going on the Collector ship. With 6/6 upgrades, I was at 190% ammo. I put on the Cerberus Assault Armor (I don't think that the heavy weapon ammo pack for the regular armor is available for sale until AFTER the collector ship mission), and hit 200% - AKA, two shots of Cain lovin'. FYI, two shots from the Cain WILL kill the Praetorian on insanity, so you can end the whole fight in about 15 seconds if you wish. You really don't need two shots though. If you only have one shot, the best course of action is to wait until quite a few enemies have come into the room with the Praetorian, THEN fire the Cain. You will take out all of those pesky husks & collectors in addition to knocking off a bit over half the Praetorian's health. You have a few more mob respawns to worry about, but the fight is much more manageable. Also... if you have the Cain and can't quite get to 200% ammo capacity, there IS a heavy weapon ammo in the fight area. So, as long as you have (I would guess) 4of the 6 heavy weapon ammo upgrades, you should be able to get enough for a second shot after hitting this box. Just for giggles, I also tried the encounter without using the Cain... yucky. I can't remember how many times I reloaded. I don't think it was an many as in the Horizon fight, but it was close. I recommend two shots of Cain lovin' for this encounter.
6. Grunt's Loyalty Mission. The Thresher Maw isn't tough at all for an infiltrator (even with no heavy weapon ammo). Just move from cover to cover, pick up bullets, and you'll have plenty of time to snipe it to death. To be fair, I will mention this. I do not think it is POSSIBLE for an Adept to kill the Thresher Maw on Insanity without using the Cain... or at least a TON of regular heavy weapon ammo. I BARELY killed it before time expired with my Hardcore Adept... and I mean BARELY... there was like 1 second left. I just don't think that an Adept can do enough damage to it in the time limit to kill it. I digress though. The Thresher Maw isn't the hard part of this quest. The Varren and other critters are the hard part. Grunt is great in this mission... and I hope to God you bought squad incendiary ammo for him. Your best backup is probably, once again, Mordin. His incinerate is awesome, and squad incendiary lets him do decent weapon damage as well.
***This reminds me of something. If you are going on a particular mission that benefits greatly from everyone using one single squad ammo power, DO NOT take anyone who has access to their own ammo powers, unless they are the one providing the squad power. If they have their own ammo powers, they will turn them on during the fight and disable the (much better) squad power that you had them using. Don't believe me, take Zaeed and Grunt with your on a run against the Blood Pack. Turn on Grunt's squad incendiary, and them bash your head against the wall as Zaeed repeatedly overwrites it by activating his own disruptor ammo power.***
Anyway, Grunt and Mordin are good choices, but no matter how you set things up, you're going to have to do a lot of running for your life in this fight. There are enough of the Varren in the last wave that it's very tough to kill them before you're overwhelmed. The Rachni critters are even worse in a way. There aren't as many of them, and they aren't as fast as the Varren, but they take quite a bit of killing, and it's tough to kill the old ones faster than the new ones can get to you.
7. N7: Blue Suns Base. The only hard part of this mission is when you're sealed in the warehouse with the two YMIR mechs... and that's only hard because you don't have any good defensive ground to retreat to. You MUCH burn down one of the mechs before they can close to pointblank range with you. It can be dicey moving to new cover with one mech on you, but it's really damned hard with two of them. I have had friends tell me that they had no trouble at all with this mission because the mechs stop a few feet from their cover where they can be worn down with repeated incinerates. This has not been the case with me. The damned things chase me all the way back to the doorway. Maybe there's a particular spot you have to get them in?? In any case, this mission wasn't a nightmare or anything, it was just noticeable because I did have to reload more than once here.
8. N7: Quarian Crash Site. No idea. I gave up. This is, without a doubt, the hardest mission in the game on Insanity. It isn't that hard to stay alive... I just can't keep the freakin' Quarian alive. The Varren make a beeline straight for him, and they turn his immunodeficient ass into a Scooby Snack in about 3 seconds. I think I tried this one a couple dozen times before I decided that a non-essential mission just wasn't worth the aggravation. I DO have a theory I plan to try out, but I'll say more about that in just a minute.
9. Derelict Reaper Ship. Or... as I like to call it, "Husk Jamboree 2010." The Scions are not a problem on this level if you're smart. When they show up, run like hell back the way you came and spam your "follow me" group command to keep your allies alive. Once you get a long way from the Scions, kill the pursuing Husks at they trickle in, then creep back up until you can see the Scion(s) and snipe/warp/incinerate it down from max range. No... the Scions are not what make this level hard. The Husks are. Interestingly enough, husk killing was one of the few areas where my Adept was NOT gimped on my Hardcore run. For whatever reason, singularity isn't blocked by Husk armor, and it pretty much kills any husks that it hits. I don't know why Bioware made it work that way (perhaps they were trying to compensate for Adepts sucking so badly against everything else in ME2), but I am grateful that they did. On my infiltrator, though, I was worried. My original plan was to just go in with Grunt and Zaeed and brute force the hell out of the husks. Well, a note on the ME Wiki that I came across gave me a new plan. The note said that husks were susceptible to Cryo ammo... and freezing them would simply make them explode. So... I respecced into squad Cryo ammo for the Derelict Reaper mission and changed Heavy Incinerate to Incinerate Blast in the process. I initially took Samara and Zaeed with me, but I reloaded the mission after I became aware of Zaeed's ammo power overwrite issue (see the note above), andI took Mordin and Samara instead. I had given Mordin Heavy Incinerate instead of Incinerate Blast, but it was not biggie. I had, luckily given Samara Area Reave. The mission was a freakin' slaughter. Husk armor protects them from the freezing effect of cryo ammo, but their armor is very light. Any husk touched by Samara's Area Reave, my Incinerate Blast, or Mordin's incinerate lost every bit of their armor. Once it was gone, one single bullet turned the husk into an ice sculpture that promptly shattered. Hell, I didn't even have to shoot anything. I just directed armor-removing powers as necessary and let Mordin and Samara do all the killing for me. For the most part, I do not recommend infiltrators picking up Cryo ammo higher difficulties of ME2, but it's an absolute Godsend for this mission. Oh, and FYI, the Cain WILL one-shot the drive core thingy at the end.
10. The Occulus. I hate this freakin' fight. I really, really hate it. Cover blows, the baddie hits hard, and LOS for group powers still manages to suck. Very annoying. The only advice I can give is just move to new (crappy) cover as much as you can, and hammer the bejeezus out of it with the Grenade Launcher or Collector Particle Beam. No Cain here... I haven't been able to use it without the blast killing me.
That's it. I don't think that any of the fights beyond this point qualify as difficult. With 200% ammo, you can just two-shot "the terminator" at the end, but there really is no need to do so. He isn't hard to kill at all, and he's actually a very fun fight for an infiltrator.
Now... here's something I was thinking about when I started writing this. Does anyone know if Cryo ammo works well on Varren? I was considering going back to that Quarian mission and giving it a shot with Cryo.