The best weapon I found was the two handed swords that are the in the top row of the vanguard class. The best map I found for this was the Colosseum map as there are fire traps that can set your opponent on fire.
I suggest that you try for this achievement in the low level servers due to more experienced players not being so easily tricked into getting set on fire by the traps. You can use kick to push the opponent back hoping that is enough to get them set on fire if the position is right.
After the opponent is set on fire you need to swing for the head, that hit also needs to be the killing blow, however with the fire and a two handed sword to the head, it has an extremely high chance of decapitating your opponent.