when you are in africa, after you have defeated the princess, you may fight in the arena.
it is really easy and you'll have plenty of skill by then, so piece of cake.
there are 5 rounds, 3 x 2 african fighters, then 4 and in the final round there will be 5 fighters.
do not use any kind of magic, or you'll not get the achievement !!!!
if you still have problems defeating them, you can also "walk away" from the fighters when the round starts (walk to the back) and then lure them one by one.
but i dont think that will be neccessary.