I put Belphegor, who doesn't have a self-heal, in the front row and she usually dies before Shiva (who I put in the back row). Their max HP is similar. Souls have higher HP and passive DEF buffs so they tend to be sturdier. My third frontliner is Triton, who has a spammable self-heal + ATK/DEF buff. Sol tends to die about as frequently as Belphegor because she doesn't benefit from my elemental HP buffs.