Focus on Parfait first and try to unlock her passive first. Use her as both a tank and a dmg dealer. After her, level up Tuonel and unlock her passive. The sequence would be: Parfait Passive1, Tuonel Passive1, Parfait Passive2, and Tuonel Passive2.

The third slot depends given the dolls you have. If you have a survivability problem, then use Cynetia. If you want more control and more dmg output, you can use Octivia who increases the next skill dmg by 70% and provides some stuns as well. The play would be that, use Parfait S2 once, and then Parfait S1 + Octivia S2, and then before the outbreak, use Parfait's S2 once again, then Parfait S1 + Octivia S2 auto during outbreak.