Your baddy should have an event that is called when it dies.
If you're using a classic system track the last person who hit the baddy using was hit (no space).
PHP Code:
function onWasH1t() {
this.last_attacker = player.account;
}
When the baddie dies use findplayer and add the weapon. I.e:
findplayer(this.last_attacker).addweapon("SomeWeap onName");