Proving Grounds

Summary

oh no

Arc (few): mini-bosses in the Tank room may be Arc. Harpies in the beginning can be skipped.
Solar (few): mini-bosses.
Void (few): mini-bosses and Interceptors in the tank room.

Anecdotally: it seems there were more Solar shields than any other.

Go along far left side to skip the enemies at spawn. Defeat the Unstoppable and Barrier champions as you go up.
Camping in the far left corner, in front of the spawn doors, is relatively safe.
When the miniboss puts up his shield at 50% health, an Unstoppable Champion and ads will spawn from the left.
Have teammates draw aggro from the miniboss while another goes to damage the shield generator.

Beginning the Rite of Proving will recharge your Super and abilities.
Ignovun will fire up to 13(ish) waves of fireballs per damage phase. You can wait this out. If he's in the far back on the raised platform, the fireballs can't hit you if you're back at spawn.
The tunnel is relatively safe. You can crouch on the ledge, in the middle of the tunnel, and not take damage. Enemies can go up to the sides and attack you. Fireballs can't hit you.
Anecdotally: when you're at spawn, Unstoppable Champions will often teleport and suddenly appear next to you instead of traversing the room towards you. Watch out for this.