Just go inside the tavern and turn right.
Kill the mob there and wait for gotrek or felix to kill you, after push respawn.
Go right where you killed the mob go to the corner behind the barell and go as far inside the wall like you stuck yourself to somewhere and throw axe to gotrek if you are a melee class or use your ranged moves.
Gotrek Drinks a pot when he is at 50% of his health, sometimes he drops to 25% sometimes he regens up to 75%, and sometimes he became untouchable after the phase and resets himself but you can solo him with this tactic.