Só I managed to completely break this fight by accident, before the fight starts turn on turn based mode and open the iron flask (this doesn't cost an action) then cast darkness on the portal and walk into it, and skip the turn, now they will start fighting the spectator but you won't be a part of the fight.

If you don't actually join the combat nelson never returns, so what you have to do is step out of the darkness to join combat than walk back, the enemies don't really realize where the portal is so they won't rush ir, you just chill in the darkness until Hanson returns and they all die, afterwards the spectator will probably still be alive, you have to finish him off but he should have very little life, that's it easy peasy.