Hey there. I noticed that you made a lot of mistakes in your "guide", so here are my recommended corrections:
1. Infiltrator - Quarian probably best - your job is to disable/enable devices (mandatory)
Incorrect. The job of the infiltrator (if you even bring one along; I personally don't see the point, as you could do much better taking along a Human Adept with Shockwave, for instance) is to sit outside away from the rest of the group and farm points from easy enemies like Troopers. It's also important that they lure enemies like Geth Primes towards the back door, where they'll be out of the way.
2. Salarian Engineer - your job is to place the decoy (can help players with little XP taking up enemy fire)
A Salarian Engineer is actually a bad thing to bring along. Not only does their Decoy not do any damage, but it prevents the enemy from attacking you, which I believe costs points in the long run. If you really need an Engineer, it's best to bring the Quarian, as lore says they're good with machines and Geth are machines.
Stay in both covers, SE & TS in the one next to the side door (C1) for the decoy, AA in other (C2) covering the enemies coming down ledders from landing pad and attempting to enter the room. Infiltrator, stay in the middle close to back door, you can revive if someone goes done.
This is bad advice. You need to stand in the open doorway. Your Salarian Engineer (you actually brought one along?) should not be casting Decoy and should just shoot enemies with his sniper rifle (I recommend the Incisor). The infiltrator should be outside and ignore when someone falls down as they have medi-gels for that.
Devices - only the infiltrator goes for these, others stay in place and shoot. Run to device cloak, enable.
Done correctly, they won't even notice you. Revive self if needed, the team should not brake cover to revive you.
Common mistake, everybody runs for devices, Geth spawn from everywhere, team dies.
This is also bad advice. The infiltrator needs to stay back and kill guys for points. If he does go for a device, he needs his teammates to help find guys for him to shoot after disarming, so they should stay close and he should avoid cloaking whenever possible.
Targets - stay in cover, they will come to you.
CM, a cowboy runs for the target , target stays on him, time out/ he dies, team overwhelmed.
CM2 breaking cover to get a piece of the target neglecting your watch, a trooper kills you from the flank.
I disagree. You need to take the fight to the enemy; that's what Sherperd would do. Kill everything, but save the marked guy for last, as he'll be defenseless without his soldiers backing him up.
Extraction - stay in cover until 40-30 seconds out then run for LZ. Or suicide after objective on wave 10 for credits only.
You should actually fight in the extraction zone so that there's lots of enemies to get points off of. You should also try and get as many of your teammates killed as possible, as the Lone Survivor will get all the points in the end (because he was the only one left to give points to).
Anyways, I hope this helps make your guide better.