After digging around on this site for some time I found a build that HELPED me play a better rumble, but I tweaked here and there and came up with this build. Im keeping this guide short cause some of these builds drag on FOREVER.

Now I know its taken me quite some time to gather all my runes so don't sweat it if you don't have this exact rune set.

With that said these runes make you REALLY lethal early game. So if your a bad early lane you might consider a more defensive set of runes.

Magic pen marks. Don't need any explanation there.

Base armor seals. Some people prefer to use these seals for an offensive rune. Ive found that this early armor helps alot.

Cool-down reduction glyphs. Alot of people have issues with this choice because it "causes him to overheat alot". Personally ive gotten used to the pace of spell casting, but if you find that your dying alot from being overheated in a time of need slow down your casting and dont cast the Electro-Harpoon. (Electro-Harpoon should only really be used for chasing and harassing)

Health quintessences. You need this health early game to be the annoying little prick that riot intended you to be.

Ive tried alot of masterie combinations on rumble. I find the defensive AP build the most effective. People dont know this, but rumble is the tankyest caster in the game. Use that to your advantage!

I start out with a Dorans shield for the armor and health boost. The bonus regeneration will keep you alive till you can go back and buy the hextex revolver and the basic boots.

After a few kills you come back and finish the mercury treads (having tenacity on rumble is invaluable) and hopefully finish off the Will of the Ancients as well.

The crystal scepter is pretty much needed on any AP hero. Makes you harder to kill while making it easier for you to kill.

Now. Depending on how the game is going and the hero's your up against you can get the void staff or the death cap. The death cap is better if your already doing good (im usually about 16-1-12 by this time). If your not doing so well the void staff will change that.

the *last* item is the hourglass. Now ive only had three games last long enough for me to need to get it, but it lets you dive in and do TONS of damage, then right as you overheat BOOM, invincible.

Max out flamespitter ASAP. Its your main skill. It does a HUGE amount of damage if they stay in the flame the whole time and it enables you to farm creeps with ease with the 2 second cooldown.

The shield is a weird skill. Its not really meant for escape, but it has saved me a few times. This skill ALSO has a 2 second cooldown. I use it in conjunction with flamespitter like this: run up to enemy hero hit W (puts you right next to em) then hit Q. Rinse and repeat. That is the bread and butter of rumble.

His Electro-Harpoon is a chaser skill, as it adds a slow (which stack if you hit both on the same target). I use it for early game harassment just to keep the health low on the enemy. Hard skill to get used to, but it does have a cool bonus. After you cast the first Electro-Harpoon the second one doesnt add any heat for 4 seconds.

His ult is amazing. Use it any chance you get. best part about it? you can cast it anywhere and going in any direction (within the parameters of the skill). Its a GREAT skill to start or end a fight. With a pretty low cooldown and the massive damage and slow it puts out it can change a chase on you to a kill for you.

Pros:
MASSIVE damage output
very tanky caster
can 1v1 almost any hero
and best of all. NO MANA!

Cons:
Hes pretty easy to kill if focused (same for most heroes)
Takes a small amount of patience on the spells so you don't overheat
hes kinda slow hero