Doom wins. He is real fast with the Magic and possesses some of the most ancient of the Dark Arts. Even if grappled, his magic would come through. If Namor were to swing hard and fast, it may distract Doom but then Namor is way too simplistic in abilities and in action (Super-strength and Flight). Doom wins by this factor, too. Namor has really no prediction on Doom, no real experience with Doom's magic so Doom again gets the upper hand with unpredictability.