Both he and the Plague Doctor's focus should be on the front two characters - the Flagellant especially, as his health and stress will fluctuate greatly with his moves. The Jester should stay in the back of the pack, buffing the characters as needed and reducing the stress that will no doubt be gained as the fighting continues. The Plague Doctor should be your healer, with the ability to cause blight where the Flagellant fails and provide some damage/buffs as needs be.

The Flagellant especially, as his Sepsis attack is amazing once his health gets low enough. A pretty strong team buff that is welcome in any composition and taking on any enemy, that also has the benefit of boosting the Jesters Finale. You should try to get your Leper and Flagellant to take the brunt of the damage. He and the Flagellant can be switched in spots one and two, depending on the upgrades you have chosen for them and where their attacks are best used.

The Leper is a solid choice for a main attacker, cleaving through foes with his large sword. There isn't much more you could want from a team like this. A strong choice for later game fights and especially in boss encounters where stress is a key factor.

Darkest dungeon boss team comp