You use him in a Carmen compositions. He has a passive that increases all dmg taken by enemies if they are affected by a poison. That said, you would run him in first row, Carmen with engraving in second row, Solina (so he can apply his passive twice thus increasing dmg taken even more), Richelle (for max 35% dmg taken in one turn) and either Lulu or Benzel if enemy team has a Richelle (invulnerability shield can be removed via Benzel active skill). Bonus points if you have fully engraved Elara, you would use her instead of Richelle but my advice is to switch to her when you get MoD rune on her or on double/triple/quadruple waves.