I remember these things being a pain in the arse but not as bad as they seem to be now. I'm trying to take a couple of them down with a solo sorcerer (yeah I know, not ideal given their immunity to magic). But I could have swarn that more used to effect them in the vanilla version. They are supposed to at least be susceptible to fire and acid (or so I thought). I've tried fireball, agannazar's scrocher, burning hands, Melf's Acid arrow, the only thing I can get to do any damage at all is Melf's minute meteors but even then it is a max of about 3 per hit (from the fire damge). I even got desperate and tried magic missiles, chromatic orb, spider summons - immune to everything. Any advice?

have you tried getting a sling? that's suppose to be blunt dmg right. or something like that.. not many monsters are immune to that I think.. it's just really weak. or maybe try summon monster on it, just have regular mobs take it down might be worth buying a wand of monster summoning if you are soloing it anyway.

sorry but I felt this was worth editing. apparently they are immune to most magic and ALL normal weapons. High resist on missile weapons and most vulnerable to fire acid and slashing. but the only blade weapon a mage/sorc can use is a dagger and thats piercing... hmm that is a problem. so neither of what I suggested would be any use. now that's a head scratcher. maybe just run past them and pray or animate dead count as magic weapons if you are lvl 7 that would easily kill them.

I managed to have just enough MMM's to take it down. I tried hitting it with a Staff +1 but that was just coming back immune to my damage. You would have thought the melf's acid arrow or at least one of those fire spells could hurt it but it was resisting everything.

They're immune to non-magical weapons, sleep, slay, poison (resistance), panic, fatigue, petrification, charm, berserk, confusion and paralyze. They have 125 magic resist, 100 electric resist, 100 piercing resist, 85 missile resist, 50 cold and magic cold resist, 30 slashing and crushing resist and have natural armour rating 4! That's why they're a pain in BG1.

What level are you? What are your spells?
Can you try flame arrow? I think it works against mustard jellies. At level 8 you get 8d6 damage per casting right?
also a sling with magic bullets should hit it

As soon as you get the gold, you can get the +3 quarterstaff at Ulgoth's Beard, which works well against those jellies, IIRC. Of course, you're not a melee character, but if you have to resort to going toe-to-toe, it may be worth a try.

Actually I don' think I tried the flame arrow, I didn't have one memorised and I'm not sure I had a scroll for it at the time. the MMM's worked pretty well but it each jelly used up basically the entirety of my spell capacity, I had to rest a few times.

That would kill my game stone dead.

polymorph into flind

I went back a few autosaves to tweek a few spells. they were immune to the fire arrow spell too but the staff+3 suggestion is a good one, I sneaked up to ulgoths with invisibility and was able to nail them quite quickly with a cast of summon monster on the wand for distraction.