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 build is purely for use on the 3v3 map Twisted Treeline, use of this build on Summoner's Rift will almost always end in Failure. Now with that aside, this build centralizes around Fiddle's amazing ability to 1v1 most champs early game, and nuke all 3 enemy champs at once late game. High magic penetration and a high ability power are what makes Fiddle tick, so we try to build both of those.
The runes for this Fiddle build are straight forward, Magic Penetration so tanks and casters won't block your damage, and Ability Power for higher damage. I choose ability power per level over flat ability power because of personal preference only, flat AP helps more early game, per level helps more late game.
For masteries I use a standard caster build of 9:0:21. We grab Archmage's Savy from the Attack attributes for added Magic Pen. and the go all utility for the nice little bonuses it gives.
First thing, buy boots! Fiddle is extremely slow, these boots first off give you a slight advantage when running away, or chasing, early game. Do Not buy a HP pot, or Mana pot, Fiddle has a built in heal, use it on minions if no enemy champs are near by, or it is dangerous to do so. Mana pots aren't needed, if you run out of Mana, you are spamming your skills too much, plain and simple. Recall to the base at 750gp to buy your Sorceror Shoes, this is the start of your Magic Pen. and is staple in helping damage all champions. Buy the remaining items in the order I showed, Fiendish Codex --> Abyssal Scepter --> Deathcap, I stop at the Deathcap because rarely you make it past it, most games will have ended before you can afford Rylai's crystal scepter.
The Skill Sequence is the way it is because early game you will try to rack up as many enemy champion kills as possible with Drain and Fear, your not a 1 man army so Don't be afraid of helping allies or getting assistance before engaging. Dark Wind, although a fantastic ability is of little use until late game, where it is useful in silencing and farming.
I choose ghost for the obvious chasing, and more importantly with Fiddle, fleeing, Fiddle is squishy and prone to ganking, this will help curve that too a minimum.
I choose flash, because with Fiddle this spell is amazing. You can flash away from enemies to live, but I more useful, and often time deadly combo is the use of flash with Crowstorm. Crowstorm teleports you a short distance, pair that with flash and you can traverse a large area, most enemies expect the are safe a fair distance away from the bushes, use flash to surprise them and get the kills. Also, Crowstorm is an effective tool for turrets diving squishy enemy champs, dive the turret using crowstorm, use fear and drain on the target till the are dead, then flash away.
Try to get people with Crowd Controls on your team, Fiddle is far more effective with a laning partner, you should get one capable of stunning, taunting, or Fearing. I suggest good teammates for him as: Ryze, Shaco, Shen, and My personal favorite, Rammus
Pros / Cons
Durable vs. 1 champion
High damage AoE Ultimate
Useful in teamfights
Prone to 3v1 ganks
Hard to farm with early game, kill reliant
In conclusion, I main fiddle for 3v3, this is the build I use, it works well most games, you won't win every game, but you should win most.