Well completely depends on how you want it to go.
So first thing to watch out for, players stopping him before he gets INTO the building, probably better if he has a solid head start, nothing stops an encounter if the players trip lock him before he even gets to where you planned the fight to take place.
Second thing. Should be a building that is already set to burn easily, perhaps a forge or warehouse, somewhere that burnable substances would cause the place to ignite quickly. Warehouse could work well in this case I think due to the fact that it could make the place big enough to be interesting.
Depending on what kind of hazards you want their could be burning sections on the ground, you could give them a chance to spread. Falling debris I'd give the building an initiative count and have it make random attack rolls to represent stuff falling, You could treat this either as a trap like roll, say all being done at +10 or treat it as a reflex save to get out of the way of the debris,
I think the idea of a changing battlefield to be great, flaming pillers falling, flameing debris blocking paths, floor boards collapsing into pits to the basement.
lots of fun to be had.