I just wanna make sure, how high your level must be to complete that shark chick underwater temple quest ? cuz boss fight at the end is just unfair. just one combo from them and everyone expect Namu are downed.
Level up until you unlock blessing of the blue dragon for Namu (i think its around 30-35. I could be wrong), which is pretty much gonna carry you for the rest of the game. Spam protective slime bubble, motherly slime and slimopolooza with Kiva. Get the weapon that shreds 100% of enemy armor on Purnima, and just spam aoe attacks with Namu. I'd also recommend stacking damage/crit buffs with Namu beforehand, most of them are instant cast like challenge and that one that raises her crit which I forgot the name of. Good luck!