I think most of the bosses are already strong enough, at least in the current version.
You said rabbit counts more as a half-boss, so there shouldn't be any problem with it being on the weaker side.
I'll take another look at the demon once I'm back at that point of the story.
As for the "fourth tail" I'm not quite sure what to do either, through I'd assume you'll just die to the next normal attack if you use it (if you don't have any healing items at least)
However I think a bigger problem is that most people won't be able to use it at all, since at the halfway-point of the battle most won't have 90% of HP left (and usually cant heal that much after the ultimate indicator is already shown).
Furthermore, if one still has over 90% of HP left at the 50% mark I doubt they'll need the ability that much.
It is also quite inferior to the "first tail", which only uses 50% and revives one instead (which is a lot better if the boss only hits once)
All in all it is a lot harder to activate than another skill which can save one from more (meaning not only ultimate) attacks (and the effect of "first tail" persists if one were to survive the attack) and leaves one very defenceless if one doesn't bring some good healing potions. (
I think the key-problem is that the skill is very hard to use since you have to get to 90+% of HP before triggering the ultimate so you can use the skill.
Maybe replace the dodge with strong defence and require an amount of currentHP% to lower the requirements to use the skill during combat.
Also I tried Salamander again and think it could use a single-target attack that doesn't consume "ignition" stacks.
All its attack skills (except "Tail Whip" and the basic attack action) use at least 2 stacks which can make it a little bit annoying to gather stacks for "Multi Fire Shoot" (Might be a typo, through shoot is a noun that can refer to an occasion when a group of people hunt and shoot game for sport)
Another thing, through I think there isn't much that can be done in rpg-maker, is that one can't attack a mimic again after fleeing from it (which one might want to do for the ones where no sign is provided).
I'll certainly test the changes and leave another comment once I get my hands on a version where they are implemented.
Naturally I'll also comment on other things I deem noteworthy (exploits and abilities where you usually have better option to choose from)
If there is any other way to help you with the game I'd be happy to assist.