I've played the campaign a couple of times now and I've probably shot about 200 Cell Ops in the knee and still no cheivo unlocked.
Is this acheivement glitched?

Doesn't count if you kill them. Try doing it without killing them. It also works on Ceph.

change your draw weight to low and try to do it.. and like sheer said dont kill them.

I've played the campaign a couple of times now and I've probably shot about 200 Cell Ops in the knee and still no cheivo unlocked.
Is this acheivement glitched?

i have done the same played loads times shot loads of knees and nothing....

funny thing is i got it on supersoilder difficulty and it was in the final battle. i hit a ceph(not a cell) in the knee about 100yards away and it popped

yet the trophy clearly says wound a cell in the knee