Barbarians: Do Nordri first as he is the only 3 star currently with an ice defense down and if you are lacking Frida or Arthur, then this is your next best bet. Throw him in the mix with the spirit link berserker’s fury strat and the damage numbers on titans is so beautiful!!
Speaking of Brienne though, I personally have one at +20 and I am gonna max a spare so I can hop between the two and possibly use them both when it is the trials with cleric and druid. After Brienne I would go Melia then Kvasir.
For clerics do Hawkmoon first then Mnesseus. As I have Mnesseus at +20 and Hawkmoon at +15, with her costume she does awesome! Her costume is great for titan stacking with Wu Kong if you do not have BT.
Crud, your fighters and monks are some tough choices as for fighters only hero i cannot speak for is Arman as I don’t have him. I have the rest though and I would go Gato first because he hits hard, fast, has revive, and he can prevent future status ailments for himself and 2 neighbors.
Monks I would go Shrubbear. I cannot tell you how many times I had his taunt on and if someone like Richard or anyone with a status ailment targeted him, then the withstand kicked in he eats specials for breakfast! I have him at +15 and his defense is 909!! And that is going the full defense and hp route until then.
Sorcerer is simple. Gill-Ra. She is very flipping good at +20 and if you go the defense and hp route, she is an excellent tank for 3 star rush attacks.
Paladin, Gunnar especially if you have his costume. With the costume and his normal outfit at your disposal, he is a good Wilbur substitution until you get him.
Wizard, imo go Ulmer. He does both direct damage to all and defense down.