You have to gear your characters appropriately and have in mind a "battle plan." Different people have different playstyles, and you just need to orient your skills and stats around having that idea work. The drops are random and the shops are random, but you can still orient your gear and skill choices around what works out for you. As an example, doing things like giving a character a weapon that only gives Focus (no atk) is going to hurt you unless you have a good reason for it. The game gives you plenty of opportunities where you have to make decisions, and all of those added up do matter. The game has a fair amount of different battle mechanics, and you really shouldn't be ignoring any of them; each one can be abused to your favor. You should never have to use items during a fight (I never do), so if that is something you are having trouble with, consider skills that would help prevent that.

One thing I can say is that defense is not that important for you, but very valuable to the AI. Just having more defense on them will keep them alive, and them staying alive keeps you alive in fights more. You, however, are probably going to be dealing the most damage, and also don't need as much defense. You should be able to block/quick evade/dodge most things. Orienting offensive stats and skills for the character you are playing can be very valuable.

In terms of specific skills, you should just have a general plan for your party. Ekseeds (+1 combo hit) are very useful for you, while keeping in mind your stacks, optimizing your stats, potential G-Unions, etc. Be mindful about your equipment and who gets what. I have multiple Ekseeds going on people, including an Axcell. Axcell is -1 mystic arte BG cost, and when stacked with the multitude of Battle Actions available, you can make your Mystic Artes cost as low as 1 BP. There's a lot of useless skills for the AI (stuff while defending for example, they don't block) and stuff that just isn't very good for you, but there's also a lot of good. Raw stat skills seem simple, but they're very good, and often better than a lot of the other random stuff. There's a lot of skills that are very specific in usage (+element damage) but these are great too if they fit the character and your playstyle. If you get a new piece of gear, taking some time to decide how you want to work it in can be useful; just throwing everything on is going to cause a mess. If you find something that is working out for you in fights, orient your skills and stats around making that work better. Know how fusing equipment works; often times there is more that you can do to make the perfect item than it initially seems.

It all really comes down to just staying in control of the fight. If you aren't maintaining control and keeping the enemies managed, and the AI is repeatedly dying/you are getting chased around, things aren't going to work out. I swap characters frequently during fights, command the AI to do different things (including specific artes), and more. You should have a plan when you go into a fight, what you're going to be doing, how you're going to fight each enemy, etc. Abuse the enemy's weaknesses, know which artes are going to be useful, etc.

Sorry if that was a bit long, it's just kind of a complicated answer. :P I've just sort of figured this all out as I went through the game. I'm still not finished, but I guess I must be getting close.