1. Make fusion weapons for your team mates that block their weakness if possible (their actual attack power is barely relevant)
2. Akihiko is basically the best character to have vs all the bosses you can use him against because he's the only one with debuffs, which you should always keep up, same with buffs except they're spread out more and you can start the fight with them up with auto skills
3. Always bring at least 1 source of dekaja/dekunda/mediarahan
4. Fuse auto-ma skills to a single persona and start fights with it (that's 3 turns worth of buffs for some pretty simple fusions since everything can inherit those skills and you can get skill cards for them too!)
5. A good resistance spread+abusing tetrakarn/makarakarn can make you pretty much invincible to everything a boss can do bar ailments or almighty spam
What exactly are you having trouble with in particular? There's no real magic trick to winning other than learning the bosses attacks.
Also the power door enemies resistance changes follow a pattern and you can work out what you can hit them with based on it. From memory, they have 3 sets of resistances that they change between, the only one of the sets has a weakness is weak to strike and absorbs slash. So you can 'test' the enemies until they all absorb slash, then hit them with a strike attack to knock them all down. But it's safer to take them down one at a time, because they'll only attack if you attack them first. Learn the resistance patterns and grind them out one at a time.
