The thing reflects all magic and projectiles, has a very high AC and maintains the distance by jumping. You basically have to become the frog to match that level of skill (jump spell on melee characters and hope they eventually hit, took me a few turns and a couple of revivals). I am doing my 2nd walkthough as a Ranger so may be I should indeed try to talk to it.
Er, no, that's not accurate.
As I said in the follow up post...
I just went and poked the frog with one of my save files... just Dia, my warlock. Started combat, hexed it, and then cast dissonant whispers on it, it failed its saving throw (13-1), took 14 damage and died.
It's reflective mucus just reflects projectiles, and it goes away as soon as it does one, and it gets mirror image for free when combat starts, which in larain-verse means that it has a large boost to AC, which decreases with each miss rolled against it. It doesn't have any particular defence against saving throw spells, or any other source of guaranteed damage (as most leveled damage-dealing spells are)
It doesn't seem like it's meant to be a hugely challenging fight.... what's its damage output like? The couple of times I reloaded and repeated the fight, it never got a turn where it had control enough to attack.
It doesn't 'reflect' magic at all. It reflects anything that it counts as a projectile, which includes magic missile, but doing so also dispels the buff, so you can target it with impunity after that, and it has no defence at all against saving throw spells.
This is just a case of playing smarter not harder.