There is a few things wrong with this
1.) after you plasma lock the wraith you need to hijack it by being directly over the gunner seat. Or else you will only be able to plasma grenade destroy it, or tear it apart.
2.) the totals are random. On my legendary solo run there were 13-14. 2 when they first appear, 1 at the first barrier, 4 when you have to destroy the ship, 6-7 at the very end of the level. On my legendary co-op playthrough there was only 11. 1 in the first time you see them, 1 at the barrier, 3 at the ship, 6 at the end.
Lego Batman 2: 100,000 Achievement points
TES V: Skyrim: 90,000 Achievement Points
Halo Wars: 80,000 Achievement Points
Fallout: New Vegas = 70,000 Achievement Points
Alan Wake = 60,000 Achievement Points
Magna Carta 2 = 50,000 Achievement Points
Phantasy Star 2 40,000 Achievement points
Fallout 3 = 30,000 Achievement Points
GTA IV = 20,000 Achievement Points
Madden 08 = 10,000 Achievement Points
Achieved 100K GS on lego batman 2's 100% completion achievement