Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is my approach to Fiddle.
Jungle until level 6, then start ganking.
Sunfire Cape play style: My reasoning for this is you get such a low bonus to your ulti from AP, why not supplement the damage with Sunfire Capes? You will have roughly 60+ damage from AP, but a whopping 120 damage from 3 sunfire capes. You will have plenty of magic pen with fiddle's great passive, plus runes and boots.
This is not meant to be a guide, just another build suggestion. Try it before you rate it. I think you will be pleasantly surprised.
The GREATEST advantage this build gives you is you are no longer squishy. Far from it. They will have to focus you, but in doing so their health is so diminished that your team will mop up what's left. Why no magic resist? You will be doing over 500 damage per second to your enemies, and they have to deal 3000+ damage to you in 4-5 seconds. No other caster will last this long. Did I mention you are draining the entire time?
Try it, have fun.