Just attack him with whatever you've got until his health bar is all the way down and then stake him. I wouldn't bother with the sledgehammer. He always knocks it away from you.
If your looking for a Buffy video game, there are two older games that are for the original xbox. the first is called Buffy the Vampire Slayer, but the second is called Chaos Bleeds. If your lucky...
Darla is the first Vampire in BTVS.
She appears in th first season and is killed by Angel whom she sired.
She is then brought back to life in the spin off show Angel, but is re-sired by Drusilla...