Rumble has low AP ratio so prioritize magic pen. Next thing is survivability; If you are not dominating get Sunfire and/or Force of Nature otherwise finish the damage items first. Third priority is AP and attack speed. Why get Gunblade over Will of Ancients? It works well in fights you overheat and combos well with Malady (not to mention the 150dmg slow). These items focus on doing damage, getting last hits, and getting you the Penta. I got mine the other day and ended the game with 23-5-10.