The Blue Slaver attacked with a Stab (13 damage) last turn.
Solution
The optimal play is to go for the kill by playing Bloodletting, Sever Soul, Pommel Strike, Cleave, and Iron Wave.
The Bloodletting will cost us 3 HP, but there's no other play where we lose less than 3 HP. We might try to not play the Bloodletting and instead play 2 × Defend + Iron Wave to take no damage this turn, but next turn the Blue Slaver will attack with a Rake for 8 damage and we can't block more than 5 damage, nor would we be able to kill the Blue Slaver, so we would take 3 damage next turn regardless (and possibly more on subsequent turns, because we still wouldn't have won the fight).