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Rumble is best AP with lots of health. You want to make sure u have enough ability power to kill, other wise u will just be running away barely touching them. The health will keep you alive since you have to be up close in combat.
I chose Magic Penetration boots to be able to knock people down. Rumble is able to do some real good damage to multiple enemies at a time. Mejai's will give you a lot of the ability power you will need. Since you can already slow, why not slow some more so they cant get away? Thats why I chose the Rylai's Scepter, plus the extra health it gave you. Now for some defense I chose something that gave me health along with some health Regen. Will of the Ancients will help you from getting killed, since you can do a lot of damage to lots of enemies this is giving you a lot of health back. To do even more damage, why dont you just throw in a Deathcap? Rumble will be killing fast!
Every time you jump into a battle, throw on your flame thrower and put up a shield. They will be wanting you dead for doing so much damage. If the try to get away thats when you wanna double slow them with your harpoon. If there are multiple people headed for the battle from the same direction, thats where your ultimate comes in to slow them down and get them to half health before they enter.