Just Play defensively Rumble is a bit squishy at start,
last hit and harass with ur E harpoon skill,

Dont bother trying to deal damage to the enemie atleast until you have 2 regrowth pendent
once you have force of nature you can start harassing much harder since your hp will regenerate back. Rumble is just another corkie deal great burst damage while enemie flee and you pursuit them with flamespitter and harpoon, his ult seem to deal great amount of damage i wouldnt say hes OP i believe he is well balance but he does start off pretty weak early game hes get a ton better after lv 4-6

Tanky Dps - because of his flame spitter and melee attack i believe he should be played either as a tank or tank ap hybrid rylias rod is a must get item. Last item can be Void staff, Hextech Gunblade, Rod of Ages instead the one i selection depending on your enemie.

Because of Force of nature and merc shoes enemies believe your a tank so they wont target you as much

Play Video Should only just watch the first 2 mins to get the idea.

Red - Magic Pent
Rest Full AP