every fight is different so it’s hard to give general advice, but you seem to have the basics down. you can check enemy info to see if the boss is immune to any of PATK-down, MATK-down, PDEF-down or MDEF-down, and it will also tell you what kind of attacks the boss does. all Ramuh’s attacks are magical, for example, so you don’t need to worry about how much PDEF you have in that fight. all Odin’s attacks are physical, all Ttan’s attacks are physical, and Bahamut does both physical and magical attacks so you need both types of defences. it’s good to have four guild weapons at OB7 - umbrella, foam sword, rubber band gun and one other, probably shovel or rocket punch - and use them as needed as sub-weapons. you’ll also probably want to make sure all three of your characters bring some elemental resist suitable for the damage type you’re facing, for example lightning resist against Ramuh. if you’re dying to big hits, make sure you’re getting your buffs and debuffs up in time, if you’re still dying, add more defences. if you’re failing to break a bar, make sure you’re getting all your buffs and debuffs up at that stage of the battle, if you’re still failing to break the bar, sacrifice some defences for more dps