I would think the preferred combat system will depend on what your table wants to emulate. In D&D or a war-gaming style, an independent system would be preferred to give a sense of a skirmish - you're resolving the battle as a whole, rather than exploring the drama of that battle.
On the other...
Sounds like a fun adventure in the making! Perhaps there can be two intersecting paths to the treasure:
(i) The floor-level path where the Headless Skeleton traverses, trying to retrace its old path through muscle (heh) memory to get to the treasure. If the players go through this level, they...