... You know that's almost funny? Because, you know the #1 most common complaint about solo boss monsters? "See, I made this powerful BBEG to fight my party, and on Round One the Wizard [insert spell here] and shut down the entire fight! I just don't know what to do!?"
And what are the solutions given?
Legendary Resistance -> AKA turn off round 1 spell victories
Add Mooks -> AKA make it a fight against multiple targets which you just said wizards and other casters excel at.
Wizards are weak against single targets? Hardly, they are so powerful that you often CAN'T have a single target boss monster.
No, what I stated was that
fireball is exceptional against multiple weak targets. I mean, are we acting like it's news that fireball is kind of OP?
Most other AoE is more balanced, either because it's weaker or trickier to position (i.e. no one complains about lightning bolt, even though it does the same damage as fireball). But sure, I agree that casters tend to be better at AoE than ST (again being cautious, since this argument tends to constantly devolve into fighters and wizards, without even accounting for subclass). More because of control than damage, IMO; a cleverly used
web spell can be encounter changing.
Yeah, legendary resistances are aimed right at save or suck spells. The point being? Legendary resistances are very much a thing in this game that spellcasters have to deal with. Good thing there are no legendary resistances to "great sword to the face." For that matter, there are all kinds of other resistances that mostly affect spellcasters, especially at high level play.
The example given was a dragon battle. We've all probably run our fair share of those. Given the options offered, I stand by my argument that I would rather have two fighters and two rogues rather than two wizards and two clerics (though obviously one of
each would be more optimal, which is the design intent).
Edit: Hmmm...mathing it out...I dunno. I think for sure the fighters are excellent in the dragon encounter - high DPR and survivability. The rogues are a lot weaker. With the clerics hopefully tanking the wizards could try to focus on burning through those resistances to get off a lucky polymorph on the dragon or something, though the problem will be surviving long enough to do so - dragons are smart and mobile and can feasibly kill a level 15 wizard (probably, what, 75 or so HP?) in one round, two on the outside unless the wizards are focusing their magic on survivability, but then it's a matter of diminishing returns - can the clerics do enough damage to get the dragon off the wizards? Probably not.
Let's assume an Ancient White Dragon, since that seems like a reasonable challenge for our 4 level 15 party members. Assuming two fighters, I'll make both plain old champions with modest magic for level 15 (say, +2 weapons and +2 AC). One is a tank with duelist and AC 22, the other has a great sword and AC 20). They could have lots of feats but I'll just worry about giving both sentinel and toughness, but great weapon master on the one. Won't bother with advantage, though realistically one fighter should be able to get it pretty often.
The tank is doing around 30 damage per round, doubled on the first round because action surge. The GWM fighter will do around 33 per round (honestly, without advantage GWM hardly does anything), again doubled on the first round.
The Ancient White has 333 HP, so those two fighters alone can kill it in just over four rounds, once they are on it. Meanwhile, conservatively assuming they have 199 HP (con 16 and toughness), that dragon is going to take considerably longer against them. Its breath hits them for around 47 damage on average (factoring in indomitable x2), while its combined melee and legendary attacks, focused on one fighter (averaging out the AC) will do 55 DPR. If it was just the two fighters against the dragon, odds are that it gets one unconscious before the other finishes it off.