Got it!(By only killing spectres.)
I think i know why this doesn't work right:
There are 2 different spectres,
Rocket
Launcher &
Assault
Rifle Spectre,
i killed at least 10
RL Spectres with the shield -> No cheevo
Yesterday i killed 3
AR Spectres aiming with the shield's crosshair exaclty onto them so their shots got directly back to them, needs ~6 shots to kill a spectre, and after the match i got the cheevo.