On quests.
Stack. The. Quests. Start a bunch up, and do a bunch in that area. Stop going out to each place individually, it'll drive you mad.
On that note, if you stack the quests right, finish them before it gets dark, and fast travel back to the area to cash them in, you can get mad bonuses from staying at an inn. For example, at one of the cities you can get like four quests at once, and if you finish them all without dark hitting (and Ignis being a jerk and telling you to stay off the road at night), you can cash all that EXP at once and get a big bonus after doing so. To better do this...
...take a trip in the general area. You can fast travel back to that spot without having to worry about it getting too dark to fast travel. What I did was just set the little fucker on auto drive and made it wander around, opening up a bunch of parking spots. I did some quests along the way, too because why not.
Quests are a vehicle for money, exp, and items. They aren't really required to beat the storyline. You don't have to do them all at once, so don't rush it. Once you've unlocked more of the world, like I said above, it makes things MUCH easier. The game isn't made for you to just go and hit every quest from the offset - once you drive near an area that you can fast travel to, it'll unlock. Relax.
Combine story quests with exp/ap things like fishing, cooking, and the like. Basically you want to abuse any bonuses you get in this game as much as possible.
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.