1) The characters strength is very weak but the monster HP is a lot, how the hell am I suppose to kill them. Not to be rude but can you gains all the characters ATK
2) The BMG is wonderful, Is there the list of that?
You're meant to be leveling up your items like the tutorial demonstrated, and using Ravage to amplify your Exploit damage, along with Taunt and Heal for sustain.
If you're struggling, make sure you're upgrading ALL your equipped items and then buying the next tier at the shop.