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.
Grenades are incredibly useful in bad situations or when you find a bunch of enemies. Cluster grenades set off biotic explosions, but you have to use another power like warp or reave to initialize the explosion. I have cleared out whole rooms of enemies with a single throw of cluster grenades after using reave.
I certainly think that the absolute best way to grind credits is on the gold challenge with another friend. One of you plays as a vanguard and the other uses warp/reave so that the vanguard sets off biotic explosions. But often I find myself wanting to unlock more gear quickly but no friends wants to play with me.
That is when I decide to bring out my special vanguard build and play against the geth on the silver difficulty. It doesn’t really matter on which firebase you play, but I find that firebase glacier is the easiest and fastest to finish.
Whenever I use grenades in multiplayer I tend to run out very quickly when I am actively using them or forget about them if I try to keep them for bad situations. In singleplayer grenades are easily found, but in multiplayer I often times find that they are hard to come by, especially if other teammates are using them as well.
The same system applies to thermal clips, although they get restocked faster with more clips.