For the Mortardom achievement you have to hijack wraiths on mission 5 and use it to kill 4 other enemy wraiths.
1) Kill the gunner on the turret
2) Plasma blast the wrath to stun it (not sure if needed)
3) There are two locations to mount.
A) mid/rear section (my personal favorite LOL) will let you destroy
B) The front "wing" lets you enter the driver seatClick here for video link