Go into local battle and use a Spartan (with the unlockable heavy throwing spear, although I did get this with the Spartans normal throwing spear) and with a second controllerpick an Apache (or another light armoured character). Choose the Castle Courtyard level and move the lightly armoured warrior against a wall, move into close range and use the throwing spear. (Thanks to TRICKY1976).
It can also be done just by playing through the Spartans arcade mode on normal difficulty, just throw your spears at the start of the round. Sometimes it will stick in the head and others it will decapitate it. It doesn't have to stick into a wall.