Well thematically Assassin shouldn't be overriding Unstable - if you instantly kill something that explodes, you'll still get hurt just the same as if you let it hit you back first.
I don't think Assassin is a good idea for Hidden Workshop. His entire playstyle relies on you killing popcorn easily, and you don't want to be killing popcorn here. Also, I think Vicious is not actually that hard of a badge to get in Hidden Workshop, maybe? Unstoppable is fine to get after you've killed the bosses, Specialist is a no-brainer, DING Max! is going to be easy to get because you get so much bonus EXP off the boss, Parched is easy, and Cheeky is easy.
I'm considering taking an Orc Wizard to just PISORF the two bosses and completely destroy that quest - that'll be very easy since they both have the same half, just knock Lavavein into Goodusa and you don't even have to care about the gimmick and can just level normally. I'm sure you could even do that Vicious AND Curious, to be honest, since as far as I can tell the only thing Curious changes is that Goodusa gains Riposte from you killing monsters and Lavavein gets Berserk. So yeah, that's my game plan, which will net me Vicious, Curious, Cheeky, Specialist, DING! Max, and Unstoppable - or if I find PISORF on the ground I could do it Faithless, too.
@Avatar - I cleared VT Ultradoom?! with a standard Paladin of GG. Here's a video:
https://www.youtube.com/watch?v=-LjfKMnh3WYI scummed for GG or Dracul. This run went pretty well. The Paladin is very good at regen fighting so even though I got the 3 fattest bosses, I had no problems regen fighting 2 of them and then spiking down the Firstborn. I would have actually picked Dracul over GG here, because there were no physical resist bosses - however, GG paladin is still very strong and much easier to use than Dracul, so I'm not complaining. The Platemail in particular was a great find that probably won me the run all by itself.
Again, kind of a lucky distribution of bosses, but with GG you can handle physical resists very easily, so really the only thing that could have screwed me was Bleaty, who probably would have cost me the run unless I found a Badge of Honor in the shops or something. The only other way I could think of to deal with him would have been with BURNDAYRAZ, despite his magic resistance.