I'm playing Baldur's Gate 2 classic, core rules. Party looks like this:
Charname LV 23 Fighter, Flail of Ages
Jaheira LV 15 Fighter LV 14 Druid, Club +2 Gnasher, access to LV7 druid miracles
Minsc LV 18, Silver Sword, 3 slots for LV3 druid miracles
Keldorn LV 18, Carsomyr
Jan LV 14 Illusionist LV 17 Thief, Firetooth +4, four slots for LV7 spells
Imoen LV 15 Mage, Short Bow of Gesen, one slot for LV7 spells

Jaheira has the best armor class at -9, but only 85 HP.

An example of a typical melee. Four Demon Knights rush Jaheira. In the blink of an eye, this happens:
Demon Knight rolls 4+6 = 10, miss
Demon Knight rolls 10+6 = 16, Hit
20 damage to Jaheira
Demon Knight rolls 18+6 = 24, Hit
18 damage to Jaheira
Demon Knight rolls 2+6 = 8, Miss
Demon Knight rolls 3+6 = 9, Miss
Demon Knight rolls 20+6 = 26, Hit (critical averted by helmet)
18 damage to Jaheira, level drained

That's 56 damage and a level drain, leaving Jaheira with 26 HP and vulnerable to Power Word Kill, or just another two hits. In the mean time, through melee, magic missiles, and some projectiles, I've killed one of the Demon Knights. But there's still three and also an invisible Cambion lurking in the arena. If they gang up on someone other than Jaheira, they go down even faster because nearly everything hits.

How do I fight effectively and not have to resurrect (via rod) or reload all the time? Protecting my mages from melee fighters isn't much of a problem, but the party buff options seem kind of weak. Protection from Evil doesn't seem like it would do all that much - if I understand the math right it means 40% of attacks hit Jaheira instead of 50%. Still enough for two Demon Knights to chop her health down to the danger zone in the blink of an eye

A couple things to try:

1. Have Jaheira cast Ironskins, this will protect against a certain number of physical damage effects; in your example, this would prevent her from taking any damage from the attacks (but would not protect her from the level drain). To protect her from level drain, you can cast Limited Wish; with enough Wisdom (and you can use a potion if you don't have enough), you can give your whole party temporary level drain immunity.

2. Have Jan and Imoen act as your tanks; with the right spells, they can be better at surviving melee attacks than the rest of your party.

As you progress, AC will become less and less useful, and you will need to find other ways to protect your characters. Damage resistance is one approach; using effects like Mirror Image/Stoneskin/Iron Skins is another; using Protection from Magic Weapons is yet another.

I gave it another try, with ironskins and stoneskin. Jaheira, Jan, and Imoen were unscathed, and the party killed two Demon Knights and a Cambion, but charname got bloodied badly (and level drained), so I withdrew charname. The last two Demon Knights changed their target to Minsc and killed him in a single round, hitting him six times out of six. The Demon Knights were killed shortly after.

Tried again, leaving charname behind, and Minsc to plink with arrows. Without the extra firepower, the fight dragged on long enough for the protections to wear out. Jaheira got hit with Power Word: Stun, and also somehow got panicked and level drained without taking any damage. Jan's stoneskin wore out and got smashed flat. Keldorn took a beating and a level drain but lived. Imoen also got level drained and panicked, but stoneskin lasted.

I'm not sure which limited wish gives you level drain immunity. "Invulnerability" didn't work.

Protection from undead gives you that immunity, though it doesn't last long.

By the way, if you're concerned about using up too many rod charges, you can get around that by casting Project Image and having the image use the rod (it needs to be in your quick item slots). You can also do this with Scrolls of Restoration (which, by the way, anyone can use).

try tanking with mages, protection from magical weapons

Also, get a cleric in the freaking party!

1) Looks like you have a bunch of melee. Try to spread the attackers to multiple party members, instead of letting them gang up on one. Also, be prepared to "swap tanks" if someone starts getting pounded.

2) Look at your available debuffs, and use them. Especially things like Hold Monster can help relieve the pressure. A Malison or two can damage enemy saving throws so that spells get through.

3) Get someone besides your healer as tank, so the healer can cast Heal during the fight.

4) See if Jaheira has access to Negative Plane Protection. That spell can block level drain. Look for equipment that can do the same.

5) Can Jan set some traps before the battle? That's basically free damage.

6) Don't pull people out of the fight entirely when they get hurt. Pull them to the back line and switch them to ranged combat.

7) Maybe you can kite them. Get them following someone fast, who just keeps their attention and runs around while everyone else engages in ranged combat.

8) If you just can't kill them, then come back later when you have more levels and better gear.

I don't think Jaheira has access to that spell (but arcane casters can use Limited Wish to get the same effect).

With that said, if you can get Jaheira to survive the battle, you can have her cast Harper's Call to revive your dead party members without having to use rod charges or Project Image shenanigans. Again, Restoration scrolls can be used by anyone, so if Jaheira is too level drained to cast the spell, you can use one of those scrolls and then rest. (If someone other than charname gets level drained, you can save a scroll by killing and resurrecting that character, if you want.)

I managed to survive this fight, but I'm not really sure how. I did that limited wish->protection from undead spell, does that tip the odds in your favor against Demon Knights aside from negating the level drain? Or maybe that in itself tips the odds significantly too?

I tried the fight twice today. The first time, I cast Greater Maliason twice; the first just got Magic Resistanced, the second hit a single Demon Knight. Hold Monster did not hold the Demon Knight. Power Word Stun did (after damaging him slightly), but the others still went after and killed Jaheira.

On the second try, I attempted to cast Power Word Stun with Jan whenever a Demon Knight was slightly damaged, but they kept resisting it. But Minsc got in two vorpal hits pretty early on, thinning the crowd considerably. The Demon Knights also wasted two of their rounds by casting useless Power Words on a full HP charname. During the whole fight, the monsters got off six attacks on total. The first critically missed, and was immediately afterward killed with a vorpal hit. The second hit Minsc on a 14, hitting for 15, The third hit Charname on a 15, hitting for 15. The fourth missed charname on a 3. The fifth hit charname on a 13, hitting for 24. The sixth and final hit charname on a 15, hitting for 18 and level draining, the limited wish must have run out by then. The whole time my party is wailing on one monster at a time, the only non-protective spell that actually did anything was a single cast of chain lightning which seems to have been useless anyway because it only hit an invisible Cambion who was soon killed with a vorpal hit anyway.

A wiki says it is Druid-4, so she should have access. If not, then Keldorn might.

The spell doesn't last long, so only throw it at the last second and only on people that will need it in an immediate manner.

----- There are spells that lower Magic Resistance (like Lower Resist). A cast or two of that should kill a monster's magic resistance, then a shot or two of Greater Malison should help them fail their saving throws, and then you should be clear to inflict magic damage/death.

Also, if your tanks keep getting killed, then summon some monsters to do the dying for you. Summon Monster, Animate Dead, Conjure Elemental, Summon Nishruu, whatever you've got. Even if the enemy is one shotting them, that's a round where they aren't pounding on your party members.

The level drain can have a big effect, especially if they can do more than one level at a time. It will very, very quickly snowball into a dead character.

Nope, Keldorn doesn't have any cleric spell, that's the difference between a Cavalier and normal paladins.
I doubt if druids have the animate dead spell and mages have better use for their lv5 slots, but a swarm of mordy swords can do all the tanking (as well as dealing decent amount of damage).
btw if Jaheira have her lv7 slot and over 300m exp shouldn't she has a Deva or something? Elemental princes? I've never took her in my party long enough to reach 300m so correct me if I'm wrong.
Also there's plenty of gears which can grant you immunity to level-drain effect.

Jaheira gets princes instead of deva.
PlayItHardcore:
- Greater Elemental Summoning.
- Druid.

Keldorn is an Inquisitor, not a Cavalier, but you are correct about his lack of cleric spells. I'd forgotten about the specific drawbacks of his class.

You've missed my point, which is "if your melee are dying, you can try casting some summon spells to do the dying for them. Here is a non-exhaustive list of examples".

The OP may or may not have any of those particular spells available, just like they may not have a swarm of mordy swords available, but these spells point them in the right direction for more ideas of their own.

Ah thanks. I make stupid mistakes about names all the time... English being not my first language and all. Also it's been a long time since the last time I took Keldorn in my party.
Also, the summoning part... I didn't mean to refute you, I'm sorry if I sound like I did. I just tried to put myself in the OP's position and wondered what would I do if I have the given party in my hands.

No sweat. I've never had Keldorn along longer than the Unseeing Eye quest line, dropping him off to take his family to the circus, and I've never run a paladin myself. Your correction about Keldorn not having the cleric spells at all was a good call; I was just thinking he might not be quite high enough for Level 4 spells (or whatever is needed).

I mentally flashed over mordenkainen's swords, but I was thinking they were 9th level for some reason. The sword swarm would probably be a very solid choice in this particular case for keeping the monsters busy (and getting some extra damage, too!).