How to deal w/ extreme challenge

Question

I play in a small group (3), in a long campaign (3.5y). This is my first campaign in DnD5e. We are at the 18th level currently and got a challenge, that seems impossible for me. I play a vengeance paladin (pure paladin) and the other two characters are wizards. The fight details:

• Battlemap w/ size of 120x120, water (15ft deep), and a 2-story little 10x10 building in the middle.
• 3x enemy
• All enemy has at least 3x legendary resistance.
• All enemy has at least 3 legendary action point.
• All enemy has HP between 600 and 1000.
• Each enemy has at least one 8th-level spell slot.
• Each enemy can dispel or counterspell from legendary action, without slots.
• Each enemy has the misty step spell.
• Each enemy has a multi-attack w/ 4x attack.
• Each enemy can potentially do 100 damage just with melee (swords with 4d12 damage and 4d12 elemental damage)
• Each enemy has a 15ft reach range.
• They are immune to fear, charmed, sleep, grappled, blinded, or restrained.
• Two has the size of Huge.
• They do not have to use any material, words, or movements for spellcasting.
• They have a minimum +11 on any save.
• They are resistant to all elemental damage types.
• They have an advantage against spells (sorry don't know how this ability is called).
• They are based on Kobolds Press Tome of Beasts.
• Each act after each turn, every time (basically if am I not wrong, they have 3x the action economy than us).

They so far dispelled everything or countered everything. We are at the 5th round yet, currently, we were able to do 68 damage to all, my Paladin tried to haste, but got dispelled. Tried to banish it, but got countered. The wizards are already around 25-30 hp, and my paladin is on 140/166.

Without haste, I think my paladin has little-to-no damage output or nothing to offer to this fight. They came close, hit me, walk away, out of reach, and can not do anything with them.

Is there any tactic or anything that I can do in such a situation where the enemy is overwhelmingly strong? Any good advice? I am not too good at tactics nor an experienced player.

Any advice is welcome.

(P.s.: maybe it is a totally normal challenge, but we never had more than one enemy with legendary actions, and our gm started to use kobold press material that is designed to pretty much kill players with ease I assume)

P.s.: since I was pretty tired after the first session, I forgot to add one detail: one of the wizards placed a Prismatic wall as a sphere (if I recall properly the name of the spell) in one place on the map.

Answer 1

Sometimes, you got to run

Not every fight is or must be winnable. You may expect such from 5e proposing balanced encounters, but the game has a long history of fleeing being the right thing to do when faced with unsurmountable odds. And assuming that your DM is not just out to slaughter you on purpose after so long a campaign, maybe they overestimated what you can do. If so, they'll be grateful for you deciding that discretion is the better part of valor.

What you describe looks like pretty overwhelming odds. One thing you could try (or could have tried) is using something like Greater Invisibility, because they cannot counterspell if they cannot see the caster. Or you could have tried using long range spells like meteor swam, because counterspell only has a range of 60 feet. But it seems too late for that now. And with energy resistance, spell resistance and legendary resistance, even if they could get off spells your two wizards pals would have a hard time to chew through all that.

What is more all three enemies have legendary actions, which they can use to invalidate your spells, they effectively get four actions on your teams' one action. Plus they have many times your hp. That does not sound as if it is going to end well.

So, try to get out of there. For example get out of counterspell range, and have one of the wizards teleport you out. Live to fight another day, and with all you learned about them, prepare better.

Can you divide and conquer? Take them down one by one, when you catch of them alone? Or at least try and surprise them next time, focus fire on one of them, and get out again. Lure one of them away and ambush them? There may be many things to do, as long as you don't fight a losing battle to its bitter end.

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PS: You did provide some additional information in comments, namely

We were summoned to the battleground, we can not leave it until the owner of the place (the winter fey queen) does not [sic] let us leave, or one of the party is unconscious.

This puts the fight into a different context. It is well possible then that this fight is just a plot device, and you are not meant to win it. The winter fey queen has godlike powers that can keep you from exercising your normal abilities, so if this fight has such defined conditions, it could be some kind of test, or at least story driven. It may be the fey queen wants to humble you, or wants to see how long you hold up, or if you indeed somehow manage to pull off an impossible win, before trusting you with some other matter.

You could of course still flee, given those conditions if taken literally, by putting one of the party members unconscious. For example, you could beat up one of the 25-30 hp wizards with nonlethal damage until they hit 0 hp and fall unconscious, and then have the other wizard teleport you all out.

You also mentioned that you have a prismatic wall sphere set up to hide in. If your foes have no way to take it down, and no magical ranged attacks you could try to shoot out blindly at them with magical ranged weapons over the course of about 10 minutes, and take them down this way. However they could still try to muscle their way in - 40d6 damage halfed by resistance, even if they failed all saves, is just 70 expected damage, something a creature with 600-1000 hp can easily shrug off, and in case they fail the banishment effect save, they could use a legendary save. So that wall is going to be less protection than you might hope for.