My most epic boss fight was actually my last session.
My players have the ability to alter destiny, which is a major plot point in the campaign. So does the big villain.
The battle took place on a cliff face with the elemental chaos on one side, held back by a barrier that was quickly cracking. It was them vs the big solo, mano a groupo.
As both sides clashed, their destiny altering powers began to cause havoc on the area. 3 main things occurred:
1) Every round, I allowed both sides to alter an existing game rule. However, the villain ability in this was stronger than the party (he had the destiny power longer). We got rules like +5 to acid damage, +2 to saving throws vs ongoing damage, bloodied targets gain regen 10, vulnerability 5, and a hit against a bloodied target lets you slide them 1 square etc.
The biggest rule change was that all ongoing damage became escalating damage. In my game escalating damage works like ongoing damage except its immediate. You instantly take the damage, roll a save, and keep taking damage until you pass the save. The boss had a ongoing damage 15 attack, so suddenly his damage became a lot more threatening!
2) Every round a surge from the chaos caused a massive blast of elemental energy, doing 40 damage to everyone in a line (rolled randomly).
3) The destiny collision caused pockets of reality to rupture, doing 20 irresistible damage to everyone in the area (area rolled randomly). Every round, each existing pocket expanded in size.
The battle started with the big bad showing his powers of destiny alteration. He used his reformation ability which allowed him to teleport everyone in a burst 12 area into anywhere else in the area (players had a fort attack to resist). The fighter was cast away and the big bad was left right next to the bard and wizard.
The bard would have none of that, releasing his teleport zone power, basically allowing his team to teleport anywhere in the combat as a move action.
A brutal fight began. The big bad had another power, alter ego, that allowed him to create a reality duplicate of himself. Not only did the duplicate have a whole suite of attacks, but at the end of the duration, the big bad could choose either the original or the duplicate to keep around. So if one had taken massive damage and conditions, that one would be dropped, completely erasing all of the pain it had just taken. However, the wizard kept pounding both villians with power, stunning and dazing several times.
The battle continued over many rounds. All of the chaos bubbles formed in the middle, causing a zone of complete death to anyone who entered. The battlefield was constricted every round as the bubbles got larger. The elemental chaos erupted several times hitting players and villain alike.
The fighter even used tide of iron to knock the big bad into the wall of chaos, and he rolled such a high strength check that I let him crack the barrier and release some of the chaos right into the villains face.
The battle went down to the wire. All 3 of my players were near 0, but so was the big bad. His alter ego was out, and the players had to take them both down to win.
The villain got a crit and several powerful attacks on the bard....killing him! But it was not so, for the bard has a word of life daily that lets him come back from 0....the first time in 6 months he had gotten to use that power. The bard came back, and the 3 delivered the final blow.
At the end as the dust settled, and the chaos that had erupted begun to die down, I allowed the group to choose one of the altered rules to become permanent. For all time, including any other games I run in that world, the change would be permanent. They chose the rule that states if a bloodied target is hit, you can slide them 1 square.