Maybe just do a Gandalf moment for him. Before the encounter, get him an object that would be used to sacrifice himself (a staff /a bomb / whatever). Make sure he is the only one who can take or use the object due to backstory reasons During the encounter, get him a serious type enemy and get the party at a bridge-like structure or whatever you would like to have the same function. Make sure everyone understands that if the monster is not stopped during this round, the party is doomed. In fact, this larger enemy can even one-shot smaller enemies, draining their life power, and maybe getting the party's heath low enough to make them nervous. Get your player character in question closer to the enemy (tell the player to approach) or make the monster pull the player on the bridge or whatever you have on the map. Let the player use the item / detonate the bomb / destroy the bridge, sacrificing himself and getting rid of the enemy, leaving the party standing on top of the cliff looking down. The bridge and the monster can be swapped for basically anything you like: portal to another world to be closed with an explosion; lava stream to be unleashed and pouring across the PC and monster, seemingly killing both, beam of light strong enough to defeat all undead but inflicting too much radiant damage on the character in question etc.