Despite its high level, the Slayer is relatively easy to defeat. It has a large amount of health, but doesn't do much damage. If the party has come this far in the Underworld, the Slayer shouldn't be a problem.
The Slayer attacks with two pairs of Keepers of Souls. The Slayer runs very fast however, so your party will encounter it long before the Keepers arrive. Don't forget to kill the Keepers as they may kill the Reaper, kicking you out of the Underworld.
The Slayer inflicts disease to those who physically attack him.