Workaround possibility.

(1)Is it possible in any way to break the hp limit for monsters and characters?
(2)Is it possible to give the monster more than the limit by linking the battle to a replica?
(By making the same battle happen twice in a row without exiting or showing the battle was won).

*SUMMON BLUECUBE*

1) Not that I can figure out, I may have to give Resource Hacker a try. I will say that you can set up a switch in-battle to "heal" a monster/character once it goes below a certain amount - Like healing a 80000 HP monster when it hits 40000, that'll give it 120000 total HP, breaking the 99999 limit.

2) You can set it up as a "hidden" monster, and bring it into play on the turn that the monster is destroyed. This only seems to work with 2+ monsters on the field, because without another "live" monster the battle would end immediately.

Battle between Slime1, Slime2, and Slime3(Hidden):

BlueCube kills Slime1
--Switch activates, reveals Slime3 (Same place as Slime1)
Slime 2 attacks
BlueCube kills Slime2
BlueCube kills Slime3

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You can probably see the obvious problem - you'll have to account for either monster being killed, and generate a slme based on that. If you only account for Slime1 being killied, and Slime2 dies first - killing Slime1 will end the battle without summoning Slime3.

Anyway, the "healing monster" method is the best one to break monster HP limits. You MAY want to do the same for your characters, maybe as a "shield" or something that breaks and reveals the character's true HP. Or you could just keep the characters at 9999, whichever you like.