There are many variations of the encounter, but I will list three. Feel free to mix and match elements of each.
Lake of Lava
Our heroes approach a boss-man who has made his home in a volcano. They have figured out how to deal with the environmental hazards, fought through the dungeon, and make their way to the lake of lava in the center. The boss-man waits on a island of rock in the middle of the lake, and our heroes make their way across a thin bridge to the island. The island is a circle with a radius 7.
Once the battle begins, the boss-man shows off his magical connection with the lava. The bridge connecting the island to the main land breaks away. At the start of each of his turns, the boss man takes a free action to crumble 10 squares of island away into the lava. He (i.e. the GM) chooses these squares, but they must be squares that were already adjacent to lava at the start of his turn. Any character who is standing in one of these squares can make a reflex save 15 to fall prone in an adjacent square. Once the boss-man is defeated, the island stops crumbling away, and the land bridge re- solidifies.
On A Spire
On A Raft
The raft is an 8 by 8 square. At the end of each round of combat, roll a d4 to randomly choose a side of the raft. All squares along that side of the raft break off and instantly sink. Any characters on the squares that break off may make a reflex save 15 to fall prone in an adjacent square.
Once the fighting stops the raft is no longer under stress, and pieces stop breaking off.