Some enemies in Mass Effect 3 have instant execution moves. The Atlas, Banshee, Brute and Phantom can kill you instantly when you let them get close. I have never seen the Geth Prime doing this, but I have been teleported towards a Geth Prime before as though it was going to execute me.

Instant executions are annoying, especially if you play as an aggressive class such as the vanguard. There hasn’t been a time when the Banshee didn’t stick her arm through my stomach when I charged it. The Brute and Atlas seems to do it less often, and the phantom rarely kills you with an instant kill.

If you are the one being killed you can’t do much about it. But if you witness one of your teammates being taken down there are a number of things you can do to stop the execution. From what I can tell if you stagger the enemy it stops the execution. The staggering is a little unreliable, sometimes if you charge an Atlas with a vanguard it staggers, other times it doesn’t. But so far the following seems to be very consistent in preventing instant deaths:

• Killing the Enemy (rocket launcher etc)
• Biotic explosions
• Grenades

From the list grenades is perhaps the least consistent, although so for it has worked probably 9 times out of 10. But obviously the rocket launcher will prevent the impalement every time. Biotic explosions haven’t failed on me yet. If you don’t know what they are or how they are caused you can read this article which explains everything to do with biotic explosions. What that article doesn’t mention is that cluster grenades can also set off biotic explosions, but can’t initialize them.