After about 2 hours of trying I finally beat that Mischievous Asshole in
. Titan Stamp screwed me over more than anything else he had; that and I couldn't Topple him. I ended up using SharlaxReynxDunban with tons of Blaze Resist and other Ether Defense stuff. I took all offensive Arts off of Sharla too. I just sat there healing Reyn and Dunban over and over, granting debuff immunity, shielding whoever needed it, and so on. What was most important was keeping the cursor over Tranquilizer. Anytime he was about to do Titan Stamp or the flamethrower attack I shot him in the face before he could get started. Did the same thing just as the battle started so that he'd be forced to cancel that flamethrower that he always leads with. Worked very smoothly.
Any other tips for the beginning other than not selling things, using many characters and so on?
Once you make some progress you can absolutely sell weapons and armor that you aren't using. Maybe keep a few extra just in case you want to experiment, but it can give you some extra cash.
If the shop is selling Arts Manuals, buy them if you haven't already. Always.
All characters gain EXP, AP and Affinity Coins regardless of whether they're in the party or not. That means you can switch out characters whenever you like, or every 10 hours and it will be okay. The only thing characters not in the active party won't gain is burst affinity (pressing the B button at the right time as the larger circle crosses into the smaller circle.)
Don't forget Skill Links. You won't be able to do much at first, but later on they'll allow for greater customization.
Slap gems onto those weapons. They're unbelievably helpful.
You can rearrange your arts in the Arts menu (not the in battle menu) with Y. So you can group things together if you tend to use certain ones in conjunction.
Accept quests as you see them, but don't tax yourself doing them. You don't need to do any to make it through the game. Similarly, as long as you're fighting most of the things you find, you won't need to do any grinding.