At last, I completed true route on highest difficulty. I was right to start from this instead of trying to save heroines, because already half-way through I was highest lvl. What I could say, this was very good game both gameplaywise and storywise. If you compare to games with similar plot, then I liked it more than Undertale, lol. I still haven't saved all heroines and missed at least three of them (Griffy, Sea Cow, Hatter) because at first run they were killed before I met them and at second negative SEN I just decided to kill everything, lol. Of course game is still somehow unbalanced, some weapons are too useless, also the fact what you could use items as a free turn unlimitedly makes standard heals and regens useless.
you can always turn up difficulty if you feel yourself too op, I am lv999 and need to rely on color souls to up my stats and all this high difficulty boss is kicking my ass hard,the ring to avoid critical is such a godsend
Strange, I never used this ring on highest difficulty, only against Crab who have instant kill attack. At lvl999 you could already have 3 or even 4 actions per turn. If you use sword which gives you +100% action then you should know what it gives you bonus action on next turn after using the sword - it means you should just equip it at your last action for it to be active. In case with 3 actions you could use any weapon first and second time and then equip the sword at third. Also damage of any skills based on last weapon used, so often it is better to sacrifice bonus action for some very heavy damage by equipping either Bow for crits or Heavy Blade for double attack (it more than halves your agility, so be careful).
If you use Knight's Sword ability which doubles your HP and Thief Dagger's skill which gives you dodge and defence +agility, then even strongest bosses wouldn't be able to significantly harm you (Jabberwocky is exception, her damage based on hp%). Late game at highest difficulty is about how to utilise multiple weapons with multi-action. BTW for highest damage it is necessary to kill JabuJabu because the bow you could get from her soul gives you very OP buff for damage (madness status + hp regen), it basically doubles your damage which is real godsend.

And strongest accessory what gives you bonus action could be gotten from Mushroom Village at negative SEN, it was real game changer for me (no longer I need to always re-equip the "action sword").