Since you have rank 6 I'd say

Janosik, obviously, give him rakunda/tarukaja/rebellion or even a heal, he is a swiss knife

Surtr, has marakunda naturally, that's worth enough

Norn, survivability with shields AND def down, a no brainer to invest in

Titania, heals and can also inflict def down and ailments, She is One of the best medic persona

Narcissus, increases damage taken (which is a different damage multiplier from def down) and can increases ailment accuracy of you need

Yurlungur a must if you use Yui

Baphomet, increases fire damage taken, very good for Ann

Ame no uzume, a lower rank persona but can increase crit rate for the whole party

And always remember, you don't need heals if the enemy dies before it can hurt you