I ran a black dragon encounter recently. Here's how it went down. TL;DR not being able to target your foe sucks, water sucks.All dragons are masters of hit-and-run combat because of their high fly speeds and devastating breath weapons. Black dragons are masters of stealth and have blindsight and live in swamps, so there's no reason for the dragon to ever come out of the murky water. He should pop up, blast at least 2 party members with Acid Breath, and then dive back under water while waiting for the breath weapon to recharge. The water could be so filled with silt and muck that darkvision won't penetrate it. While the dragon is underwater, he should rely on lair actions, and if he's a spellcasting dragon or has any magic items he can use, he should spend a few rounds buffing. (I'm a bastard so I had my black dragon cast greater invisibility on himself.) Hiding is an action, so if your DM is running things properly, everybody will know where the dragon is until his next turn, but he'll still have total concealment from the water. If possible, the dragon should also have some underwater terrain to go behind in order to get total cover.
If anyone does engage the dragon in melee underwater, that character is probably at a disadvantage on attacks for lacking a swim speed and not using lame weapons -- most PCs are just not prepared for underwater combat. Plus, that character will be taking the full brunt of the dragon's legendary actions. Divide-and-conquer is a great strategy, and a smart dragon (and they're all smart at that age) will voluntarily accept Opportunity Attacks in order to move away from melee combatants. It's much better to force your opponents to use Readied Actions to make single attacks than to have access to their full multi-attack routine. Black dragons don't have any particular skill at grappling, but they're pretty strong, and getting grappled and dragged around underwater is pretty brutal. A shrewd dragon will also have some underwater minions, preferably also grapplers or with swallow-whole (my dragon had a few pet "devil sharks," a Huge nasty shark from Tome of Beasts). Most PCs will try to stay on dry land, so a clever black dragon won't have much of it in his lair, and will shape the land to suit his hit-and-run defense style. (My dragon had all his treasure on a single island in the center of a cavern -- and the island was just small enough to fit within the radius of the darkness lair action, so everyone on it was blinded every other round.)
An older dragon has probably seen humanoid adventurers before and should have some idea that the paladin and barbarian are high-HP melee damage-dealers to be avoided. If your DM is as nasty as I am, the dragon will focus his Acid Breath on the squishy-looking members of the party first. The dragon's blindsight should neuter the assassin rogue. (Well, it's questionable whether a regular humanoid can hide from blindsight using the regular old Stealth skill. I'd discuss this with your DM before it comes up.) For the dragon, getting any of these squishy characters into the water for a right legendary thrashing may also be a good move, worth a fly-by grappling on a round when the breath weapon is unavailable, or a swim-by attack if the character has been pulled into the water by a lair action.
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...I'm wondering if this is perhaps more detail than you were looking for, and possibly spoilery. Anyway, here are some spells I'd recommend.
- If anyone in your group can cast control water, this will absolutely destroy the strategy I posted above. I think maybe the druid can prepare it?
- Water walk and water breathing can both be very helpful against a watery lair.
- Tasha's is actually a great spell here because if it works it can immobilize the dragon, allowing melee characters to dash over (using water walk) to issue the beat-down, and it's only 1st level.
- Faerie fire is another one that several characters could spam to neutralize the stealth aspect early-on. It's also 1st level, which is important because the dragon has VERY good saves, plus Legendary Resistance, so you don't want to burn your higher level spell slots until you've got a clear shot.
- Against a spellcasting dragon, I'd rate dispel magic more highly than counterspell because such a dragon is more likely to be using buffs and ongoing control spells than direct-damage or save-or-suck spells. However by 12th level you can probably afford to prepare both.
- I'm not sure how much healing spells your druid and paladin typically carry, but they should each have at least cure wounds for this fight. A single good damage roll on Acid Breath can really ruin someone's day.
- If you've been saving up any magic items for a rainy day, that day is today.
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