You can solo all 4 tiers, just need to stay near blunderers’ spawns and not to aggro vets. It’s rather easy as a zerker as you can kill them in a few seconds and it’s best to have cc for the blunderer at right if you dont have that great ranged dps.
It’s rather easy with thief as you can stealth when vets spawn next to you and back a bit until they run away, I suppose it can be managed with any other class too.