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 was using the drell adept when I was focusing on the cluster grenades. I have written a guide for the drell adept, which you can find here, but in that guide I was focusing on making the drell adept the most reliable and best class in all challenges. Unfortunately on gold and to some extend on silver this tactic isn’t going to work very well because of the grenade supply system. I have also written a guide about that here.
When you level reave focus on area of effect. That is going to be the most powerful along with the increased radius, force and damage of biotic explosions. With the cluster grenades focus on area of effect, number of grenades you can carry and shrapnel count.
Once you can carry three grenades at a time at the start of the round head towards the area of the map that isn’t occupied by your team because that is where the most enemies will spawn. Use reave and immediately throw one set of cluster grenades at the reaved targets. The enemies will fly all over the place whilst dying. I found that I was having a lot of fun doing this.
The only problem is that the grenades aren’t thrown out in the same way every time. Sometimes they one will go in the middle and two on the sides of your cross hairs, but often times they won’t go anywhere near the middle of your screen. Be sure to be close when you throw the grenades, the explosions can’t damage you.