On combat
Chill out. Stop mashing. You can hold the button down to attack, sure, but canceling out will give you stronger moves sometimes (and some that can even stun an Iron Giant, like with the gun). I like to tap it if the enemy is fast and the weapon has a lot of lag time.
Parrying/blocking can be done by just holding it down. At least I'm pretty sure.
Back off from fights. You aren't supposed to wallow in the middle of it. Stay mobile, warp out, dodge out, whatever.
Use techs. Unlock moves for you bros and if you have enough bar, you can chain them together (i.e., Glad does his downward smash thing and then prompt does his fireworks, and then Glad does his shit again).
The game is eventually going to stop pissing around and holding buttons isn't going to work. That's fine, because you need to, again, stay mobile and strike from behind. Basically, the game wants you to flank dudes as much as possible.
Use spells. A lot. You get a lot of shit to craft them and they can clear out bosses and things higher than your level if you do it right. Freezing enemies particularly works well.
Enemies are weak to certain weapons. Things like Flan aren't weak to shit but magic. Ignis can analyze things as a skill unlock. You'll want that.
Look at your weapons. Some of them have special effects. There's a cave that has a bunch of shitty Flans that are weak af to fire. There's a sword that's a fire sword. This hurts the Flan.
Eat. Cooking bonuses and eating at restaurants are more than worth the money and allow you to tackle things you shouldn't be able to. Abuse that shit.