Rumble's kit works very will with flat pen, and magic pen in general. You can reduce enemies magic resistance to sub 20% and still be tanky enough to sit in the front of fights if you build correctly. math time.

Enemy Sejuani jungle builds MR all the way to 150~ MR. This is a standard amount of MR.
Rumble builds Sorc Shoes, Void Staff Liandrys and Abyssal (along with Zhonya and Rabadons).
150 x 0.65 = 97.5
97.5 - (20+15+15) = 47.5
47.5 MR = 22.2% magic resist.
150 MR = 60% magic resist.

look at how effective these items are at increasing the damage you do, and especially be thinking about how exponentially armor works and how a mix of flat and % pen can maximize your damage and shred on a target.