Hijacker Achievement in Mass Effect 3
Hijack an Atlas mech.
How to unlock Hijacker
To hijack an atlas, you will need to focus fire on the yellow glass on the front to shatter it and expose the driver. This should take 8-10 shots with a sniper rifle or more with a weaker gun (less on a lower difficulty level than Insanity). From there, kill the driver. Once he is dead, simply run up to the stationary mech and hop in.
I personally got this during the From Ashes DLC in the final battle. Does not count by jumping in an already abandoned mech! You have to personally kill the driver and take over one that was previously attacking you.