It is highly recommended to use the Protect from Melee prayer before speaking to him and and accepting his challenge, as he can hit up to 47. Although Keef can hit through Protect from Melee, he will deal far less damage if it is active. As Keef is a large NPC, you can use the nearby tree, for example, as a safespot. When he is defeated, Keef will beg for mercy and agree to not help the monkeys.