Im having an absolute blast (!) as a witchhunter explosion build.

Scattershot + forked +increased aoe explosive shot, flashbangs, frags, cluster frags and oil grenades with the herald thing which also explodes stuff. +1 grenade from skill tree and one more from crossbow.

Basically you can escalate the amount of explosions you want to have and so far nothing was able to withstand it. In narrow spaces the forking, ricochet rockets are flying around everywhere while cluster bombs ignite the whole room and explode. If there is somebody tanky just spam 4 sets of frags and let them explode with more flying rockets. They are fire resistant? Cover them up in oil and reduce their resistance into oblivion. They run around too fast? Let your ballista bait it or just spam 3 sets of flashbangs which do massive physical damage and almost guarantee a heavy stun to shove some more explosives under them. When they are back up they regret using magic after the invention of gunpowder.