To me this idea is playing out backwards. It feels like the party has hunted Hoarfrost to this dead-end place and he is making his final stand, not that he has hunted them and led them into this trap. I would think instead of being in the chasm, he would leave the girl staked out to lure the PCs in while he and his minions are somewhere on the outside waiting to trap the PCs in the chasm. This puts the PCs into the "final Stand" position, fighting for their lives with the menace who has stalked them across the land. Couple this with a mix of traps and hit and run ambushes while en route and this could make for an interesting encounter. As it is, it has the makings, just in the wrong order.