Daiji (Last boss) isn't that difficult if you properly use buffs and respec. Anegorami has even done a video here showing him doing the entire thing in 3 minutes on NG (with the Magnum, I'm guessing...):
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In general, the only thing that's not immediately obvious is that using a Lampas Drop in the 2nd phase will make you immune to the pollen attacks, making that whole phase a breeze. In the first phase, if you forego dodging her followup off of her temptation and instead use one of the Psi Wind skills, she'll get knocked off guard, giving you a free turn to hit her. Use a Charred Salamander to respec all the points into useful perks, Auto Stamina is a god send on these fights no matter what build you're using. Agility up is nice too to make your attack hit more. If you use buffs, Cool Calm is great on the first phase, the 2nd phase is free, and Sylph Storm will dodge almost all the attacks on the 3rd phase. 4th phase is incredibly easy if you're good with the dance mechanics, she basically can't hit you.