The "Hijacker" achievement requires that you gain control of an Atlas mech. What you have to do, is equip yourself with a powerful weapon - I prefer a sniper rifle that fires more than one round per clip- or atleast one that gives you a constant rate of fire; pistols, smgs, and assault rifles, are a good alternative. I recommend using an ammo ability like armor-piercing or warp ammo. Then, continue to fire at the canopy - the yellow shield protecting the cockpit - while maintaining decent cover. Eventually, you'll notice it begin to crack the more consistent your shots are, until it eventually shatters and kills the pilot inside. Simply run up to the cockpit of the Atlas mech and climb in to gain this achievement. Most of its health will probably be gone, but you should still be able to deal a decent amount of damage first.

Note that you cannot gain the "Hijacker" achievement any other way, such as in the Grissom assignment; you have to be in active combat against an Atlas mech. There's plenty of missions, especially side missions for you to earn this achievement, so don't worry too much.