Sigh. Anyone won this fight?
I tried like a dozen of times and no luck, I sometimes manage to have only one left but they
often "call allies", so I ended up killing 4 in total while at the moment fighting 5! ...
They're immune to any effects and cast super HP absorb/call allies very often when low on health.
It's really insane how some JRPGs are unbalanced, all fights so far were trivial but this one seems
impossible to win, no matter how hard I try.
I had to look them up in my monster guide, I didn't remember any tough fight in Rolent off the top of my head. But I played on normal difficulty, tend to overlevel my chars, and managed to grab couple of tiger hearts/crimson eyes in chapter 1, your situation can be quite different, so I can only offer some generic advice.
Avoid the fight for now and go gain level or two. If you find shining poms (I'd say where they roam, but you didn't call for spoilers) it shouldn't take too long - the ones in Rolent are level 60, Mouki are only 59, there should be some reserve. This game expects some amount of grinding, that's very common in this genre.
Or use the "dumbing down" game feature ("retry offset" in options menu), loose the fight once and try again with easier version of opponents.
When you fight enemy that splits or calls for help when low on health, focus fire on only one of them. In this case both literally and as a figure of speech - they are vulnerable to fire and earth. Estelle's S-break fired with 50% bonus can really mess enemy hp. If you have ruby or topaz talisman (I don't remember where I got them, at the end of game I had two of each), put it to use too. Agate's s-break is pretty good too, if you have him with you.
The fight is definitely winnable, only fight that I had to repeat several times was the timed fight aboard Glorious (which you can loose anyway, at the costs of 1 or 2 BP). And it appears that whole Rolent sewers are optional, you will only miss one entry in monster guide if you give up, no BP lost.