Survive all five waves in the battle for Object Rho.
This one is missable, because you can get knocked down but won't actually die, and the story will go on. So if you don't want the achievement, just let yourself get iced immediately. But we aren't here for that, are we ?
You have to survive 5 waves of attackers, so if you get downed, immediately DASHBOARD OUT.
Luckily, I found an exploit to win the fight easily. At the start of the fight, do not stay there but go to the right, up the little stairs and take cover behind the side of the console here. You should be behind the console's left side if you look at it, and have some glass cover on the left, then the stairs. Now you will hardly ever be attacked from the left, and all attacks towards you will finish in your little console wall. And the best part is ? You can shoot THROUGH it, without getting out of cover. The same goes for the left side of the room according to Baihu1983 and some classes find hiding behind the "L" shaped console better for the YMIR fight because the Mech doesn't rush you there, so try both !
For those who want to play fair, beware of grenades that will knock you out of cover, and here are my tips :
Wave 1 : regular Soldiers with regular weapons. Just go hide in the corner I previously talked about. Even if there is a passage on your left, the enemy will hardly ever take it unless you come out of cover for an ammo run (they will hide behind the 2 columns instead), making it the best cover in the room. Same goes for the left side of this symmetrical room.
Wave 2 : regular Soldiers with 2 Heavy's with Flamethrowers. Theses guys can rape you, so again, use "my" cover and you will be safe. You can even detonate one guy's tank when the other is next to him and take the 2 down in 1 shot on Casual.
Wave 3 : regular Soldiers with 1 Heavy Vanguard and 1 Heavy Flamethrower. The first is a pushover and won't rush you, so eat away his armor, and ice him. Be careful with the Flamethrower Heavy because he WILL rush you.
Wave 4 : regular Soldiers with 1 Heavy Engineer and 1 Heavy Flamethrower. The first is a pushover and won't rush you, so eat away his shields, and ice him. Be careful with the Flamethrower Heavy because he WILL rush you.
Wave 5 : same, plus an YMIR Mech. Yeah. In close quarters. Yeah... If you're playing a maxed-out Infiltrator like me that can do 1000 points of damage in one shot, go into Tactical Cloak and headshot the Mech twice with your Widow and it will go nuclear, so move your ass out of cover after that ! If you're playing another class, use Power Armor or Heavy Geth Shield Boost with a combo of Heavy Overload, Heavy Incinerate and/or Heavy Warp, the best biotic power in the game. Try to use Warp Ammo as well, again the best ammo in the game. And use your best weapon, of course, depending on your class. Too bad the frag grenades from Mass Effect 1 aren't there anymore.
You have to survive 5 waves of attackers, to survive the waves just go to the left of the room when you start and go in the corner behind an L shaped desk. They wont even come near you.