Dealing with enemy types:
Regular infantry - Short, controlled bursts if possible. High damage output staggers them on occasion, allowing for an easy kill. Beware, however, as they are ruthless and dogged. They will actively flank and surprise you, using a combination of rifles, grenades, and shock batons to bring you down one by one. Be wary of engaging groups of them (or any enemy, for that matter), as there are ALWAYS more than you think.
Centurions - Fairly simple, just regular guys with shields tacked on. Get out an assault weapon and go to town. Again, staggering them by any means is almost a sure win. Slow fire weapons are weak against shields, so conserve ammo for the heavy hitters until they're down. If you lack any strong means to combat shields, Centurions are an issue. Also, ensure that you kill them before allowing their shields to regenerate, as this will happen if they manage to slip away from a firefight.
Guardians - A few ways to get past the shield. Any power which staggers or distracts opponents (Combat drone, Sticky Grenade, Pull, etc) will cause them to fall backward, opening them up for a good ol' fashioned hose down with your weapon of choice. WARNING, DO NOT MELEE THEM AS A RULE! IF you have an opening, take it, otherwise leave the flashy stuff to other enemy types. Not especially aggressive or deadly, but if a few of these manage to flank a weakened squad, they become a real nightmare, as they provide a wall for more deadly enemies to close in.
Nemeses - If you have a class with Overload in your lineup, you're in luck. A Nemesis has high shields but very poor health and damage mitigation. One blast from any shield breaking power ought to be enough to open them up. As an added bonus, they stagger when they lose their shields. Since they carry sniper rifles, don't be afraid to get up in their face and punch them. When facing one from range, watch for the bright red flash, indicating their laser targeting. They may be weak, but they hit very hard with each shot.
Phantoms - Overload, then stunlock. Actually, any barrier destroying power will do, so long as it strips their protections away fast. Phantoms are not terribly annoying at range, but they are positively awful if they get into melee range. Their swords are lethal, and they move fast enough that hitting them is difficult in the heat of battle. Thus, engaging them quickly and relentlessly is advised. Break the barrier, and light them up. They apparently won't move if they are being shot fast enough. Also, watch out for their Tactical Cloak, they can be quite sneaky with it.
Atlus(es) - Save all missile launcher ammo for the inevitable Atlas or two in each mission. One shot is all you need to ruin its day, and generally this allows for an easy win. Buying Veteran Packs seems to give +3 missile ammo quite often. If no missiles are available for whatever reason, prepare for a long fight. An Atlus is always accompanied by numerous foot soldiers, who use its commanding presence as license to hit you from behind. Keep your squad tight, well covered, and ready to move. Use fast firing weapons like SMGs, in tandem with powers, to strip the shields. Then switch to slower, more powerful weapons and powers for the armor. But seriously, just use the Missile Launcher.
General Rules...
If your squad gets wiped out, a highly mobile, more defensively oriented playstyle is more effective for getting to the end of the round than camping. Enemies will flank a stationary target too quickly for you to handle. Furthermore, if this happens on a level requiring hacking or uploading/activating points, I believe enemies spawn endlessly. Have fun with that.
Headshots inflict more damage than body shots. Common knowledge, but worth mentioning. If you happen to main the Mattock, or similarly accurate guns, headshots are incredibly efficient. Still, the unlimited supply of ammo at various dumps makes pinpoint accuracy less of an issue than it could be.
Conserve ammo as best you can. This becomes a bigger issue when your squad is surrounded, especially by an Atlus. Running out of ammo for your long range weapon is crippling in a firefight. Ammo dumps abound in both levels, but getting to them is not always an option. Spray if you need to, but always keep your ammo count in mind at all times.
Stick close to your squad. It can be easy to lose focus and over-extend yourself (Vanguards, looking at you lol), but try very hard to avoid letting this happen. Remember this one crucial point, if you gain nothing else from my post. THERE ARE ALWAYS MORE THAN YOU THINK. ME3 has lived up to its promise of higher difficulty. Throwing yourself into a group of angry Cerberus goons gets you nowhere but dead in a hurry. There are always the moments of exception, where you do exactly this and wind up massacring the enemy. But those moments are just that, an exception. Don't tempt fate unless absolutely necessary. You are given 3 other teammates for a good reason.
Well, that's it. TL;DR or Cool story bro me to hell and back, I just stopped by to offer some of my extremely limited advice. All of this info is a snap to figure out on your own, and I'm sure most people who have played for some amount of time have picked all this up. For those who haven't yet, haven't had a chance to play yet, or are just as unskilled at online shooters as I happen to be, welcome to ME3. Any amount of frustration from the losses that are certain to come only makes the awesomeness of forming a kickass squad all the sweeter. Have fun, I look forward to playing many more rounds with my fellow PC gamers.