Blue Resort combat: look for characters that have a unique Ultimate. Ones that you hover over and it just says 300% are generic. Unique Ultimate, probably unique skills. Generic skills are bad because they have fixed elements. You want ones that use the character's element so that you can use earrings. Some examples:
Djeeta/Gran: strong party attack buff, decent single attack, strong group attack
Lyria: good dodge buff lets you ignore most boss damage, healing, ultimate spam
Narmaya: first skill is a strong self buff that doesn't take a turn, second skill does ridiculous damage and debuffs defense
Cagliostro: big all-around party buff that's less useful if you're using D/G or Lyria, heals
Prometia: strong attack that increases her Ultimate meter, Ultimate hits all and heals party MP
I beat the level 35 dungeon thing with the non-Cag characters above.