We had a rocket trooper that wouldn't die after a failed grab animation. Normally you can get them to climb cover but this one would just walk through it. After trial and error we finally figured out how to get rid of him. Get him to the edge of any map with a drop off and you need to have a grenade user. Everyone stand on the edge of the map you want him out of then once he steps out he may float outside and not move. When this happens everyone step back and throw your grenades right behind him. He will respond with a dodge. Eventually he will fall and you wont have to lose your time and credits.
It happens that the enemy is actually somewhere else than where you see him, if you find that spot you can kill him there. An example are the enemy totem poles on reactor where spawned enemies jump off the rafters and somehow get stuck on top of each other. If you target their spawn point on the rafters instead of the totem pole itself, you will kill them.
We were actually struggling with a Possessed Trooper that refused to die on Reactor. We were coming from the back through the reactor when he dodge rolled or something off a railing and into the abyss.
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