I have two suggestions:
If this BBEG would be an elite at level 22, lower him to 17 and make him a solo (brute perhaps, soldier would be REALLY grindy). It'll be tough, but perhaps doable on their own.
The second is my preference, as it worked well for me in a very similar situation. In my scenario, the players, level 13, were fighting a level 20 elder Blizzard dragon BBEG on top of a broken spire 200 feet high. To compensate, an Archfey of the summer stepped in to help counter the Blizzard Dragon. She used minor actions on her turn to give the players + 10 to attack and damage and skills, heal some people for a surge + 25, move people around, and the like. She didn't buff defenses at all.
The end result was a very swingy, very intense, fight. Players were taking 60-80 damage a round, but were being healed up to full the next with both the leader and the archfey healing the group. Their damage was pretty insane as well, as multiattacks and granted attacks added +10 multiple times. Basically, the group couldn't plan round to round with how swingy the battle was. I even ended up using a triple attack on an unconscious defender (not dying, just one that fell asleep due to an attack), doing 135 damage to him. He died the following round to ongoing damage. Then I pushed the defender off the spire. Luckily he had a flying power to slow his descent and not die on impact.
I think that battle ended with 2 of 6 players conscious. It was a doozy, and the players still talk about it a year and a half later. So my suggestion would be to have someone come in and help them. If they don't know anyone that powerful, here are some ideas (not that you'd need them I'm sure):
* Powerful being caught onto the PC's fame and wants to exploit it. Being helps them win this so they owe this character later.
* BBEG has a lackey who is actually a spy for some other entity and helps the players take out this bad guy.
* BBEG's boss helps players take out this guy, revealing a more twisted plot, and that the players have done exactly as big boss wanted in helping him take out current BBEG.
TL;DR: Look up Bahamut in his human form from the Draconomicon 2. He buffs a party so they could face a challenge roughly 5-10 levels higher than they are, and use a similar NPC to equalize the battle.