I find that lag is the usual culprit of invincible enemies. This tends to be compounded when using any ability that affects a units movement (e.g. cryo, stasis, etc.)
I find using area of affect attacks will
"sometimes" cause a unit to pop back into the right place so you can
kill it. (e.g. Assari heavy melee) I find it most useful when a phantom
gets statis+biotic-explosion glitched (demo)
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My most recent (and unique) invincible enemy (In ME3 proper, not the demo), was caused by trying to
grab a Cerberus assault trooper (because I think grabs are awesome and I would dart half way across the map just to get one

)
-A trooper was behind a wall taking cover
-I was running up to the wall
-As I was running up (but BEFORE I reached the wall) the trooper stood up and started to walk away from the wall
-I got into cover, and started tapping grab (F key on keyboard), but I "thought" I failed to grab him to I strafed
This some how caused "half" of a grab. The trooper walked back to the wall and the camera snapped into the grab animation angle (and then went back to normal as if the grab was finished) but the trooper didn't die, I didn't do the grab, and instead it jumped over the wall, an was permantly invincible.