Buffy tricked Luke into thinking it was daylight, then killed him.
Catherine Madison (Witch)
Buffy reflected one of Catherine's spells back at her, trapping her inside her own cheerleading trophy.
Buffy used bat sonar to confuse her, then cut her into pieces with a sharp blade.
Hyenas devoured him.
Angel staked her through the back.
Buffy tricked him into punching some sort of electrical box, frying him and causing him to explode.
Brotherhood of Seven
Sid the dummy killed 6 of 7, then the seventh was shoved into his own guillotine, Xander released the rope, and the demon's head got chopped off. Sid then stabbed the demon in the heart with his knife.
Billy Palmer faced his fear and removed his mask to make him disappear.
Buffy threw him through the library skylight and he was impaled on a shard of broken furniture.