Eek you misinterpreted a little bit. When you start, it's a given that you need to pray range (it is silent, and his magic has a sound). Now, when you see him, the strategy changes. He has two animations: range and mage. Range is an obvious stomp, and mage is a more subtle head-roll. Because of the fact that the headroll is sometimes harder to notice, keep mage prayer on during the fight. When he stomps (it is VERY easy to notice), switch to range, then after the attack splashes, immediately switch back to mage prayer. When he uses a magic attack, use the time to drink pots or whatever.