No idea where but if you are good enough to poke at it with a hex editor then you can seek it yourself -- such things are great new hacker projects usually.
Pity one of the hacks does so much else than the health thing as compare and contrast is so very effective.
Find the active health, rather than max, the same way you find health, ammo, mana, time... cheats. This is to say do it some damage, check, do it some damage, check.... ( https://web.archive.org/web/20080309104350/http://etk.scener.org/?op=tutorial
if you are new to cheat making, https://web.archive.org/web/20160203034425/http://doc.kodewerx.org/hacking_snes.html
might also be of interest). As you want to minimise things it can help to give your characters infinite health, mana,.... and maybe equip low damage items to give yourself more chances.
Find the health there. Depending upon what cheat options you are given you can possibly do an if greater than then set to 750 and that way still fight it down "normally" below it. Hopefully 750 is not too low and it triggers low health behaviours (some games use a percentage of max).
Mind you https://datacrystal.romhacking.net/wiki/Secret_of_Mana:RAM_map
mentions a max health value for RAM and right next to the current HP value which is nice (sometimes they are far apart, or calculated values based on stats or something). You could edit that down, and possibly one edit the current HP as it might not check.
Confirm that data is accurate.
If the max value is in RAM you can even make a cheat to hold it at that value and not have to worry about the if/greater/else constructions.
If the game was unknown to the world at large then here you would set a break on write to the area in question, load a savestate outside battle and then go in and see it. You would then go backwards from there until you found a read of the ROM and go with that.https://datacrystal.romhacking.net/wiki/Secret_Of_Mana_Bank_Disassemblies
has a few banks and each have various things dealing with some kind of max HP. In an ideal world you would compare the RAM location to the disassemblies as they would usually note what goes where (or near enough in the case of multiple enemies), however I am being lazy right now so you get to do that if you want to.