For people who are having trouble with the last battles in the answer I'll say it again: block the types of attacks used by your opponents.
There are items obtainable from certain fusions that null a certain type of attack. I had an item that nulled every type of physical attack that by itself made the battle against junpei and koromaru seem easy. With a persona strong against fire and dark I was invencible.
The same happened with the others. I didn't use any guides. When you see a fusion that will give you an item go for it, it will be useful.
Fuse your personas until you get one strong against the attacks your opponents use. Just become invencible and let metis die.
theres a problem with this. you cant become invincible against everything. in the current battle i'm at i'm either dying due to charm or mind charged bufudyne. if i put up a magic shield in an attempt to stop the bufudyne they cast megidolean wich ignores the magic sheild. making this artificially harder is the lack of a compendum in the answer as well as the fact that you start at lvl 25 making it pretty difficult to actually fuse a narcissus. wich would be the easiest way to block charm. also because of the situation you are placed in late in the game simply going to level up is not an easy task.