For the Hydrangeas, you may have to use a bomb or two (have John use them so both fighters can get criticals). If you can't kill at least one of them before they use the pollen fog move (which is the precursor to mind pollination), you might as well put the fight off until you can handle it later.
Just as an FYI, you technically can wait the debuffs out - the Mutated Hydrangea's 300 MP will tap out after casting skills 30 times (Foggy Pollination, Mind Pollination, and Draw Power: Earth all consume 10 MP), but be prepared for a long slog if that's the way you're going with it.
Also, if you haven't already you probably should pick up Pierce from Paul as it will boost your damage output.
Ed: The "split" demons only have 1 hp - the ideal situation is to have John use Slash the same turn the splits, but if John using Slash is completely nonviable you could theoretically make due with a Stasis Bomb as it will hit all enemies (though you'll probably want to keep a few for a later boss fight).