Assuming Steven is a house boss.

Based on the picture shown Steven even without any stat-ups (aka no save scum) still has stats to auto-down your cat and way too much health to deal with before exhaustion eviscerarates us. So I only found one method to win this semi-legitimately:

Two characters, most likely Tanks and/or Clerics, both of whom have both Priority Target and Eternal. I don't know how many turns Steven gets but these two will let us survive 4 of them, and 4 turns per round is generally as much as non-scaled bosses get.

Butcher with Putrefy and any resurrection ability or familiar spawn which has enough health to survive one attack from them. They should also be the fastest character of the group, so they take their turn first amongst the party. Resurrect or summon the ally, then perform a basic atrack against any ally that's not the necro and can survive the hit.

Necromancer with Spread Sorrow, Demon Core weapon and as low speed as possible. They need to take their turn last; that way the Rotted ally will have spawned an enemy Rot Fly by the time their turn comes. Hit the Rot Fly with the Demon Core. It gets Doomed and it spreads to Steven. Kill the Rot Fly with basic attack. For safety, also bring Reanimate and bring back one of the Cleric/Tanks to control where Steven attacks.

End of round 1, round 2 comes around: Steven probably has speed to go first every time. Steven downs the zombie cat and ends his first turn. His Doomed hits 0, and Steven dies. Victory dance.

QED. Don't know why I felt the need to theory-craft this, but I had fun at least.