From: IhatethisCPU | #450
Ths is probably an odd question, but how does one build Trundle?
Merc Treads/Ninja Tabi
Wriggles
Phage
Warden Mail/G. Shroud/Negatron (Whichever of these options you'll need most early
This is your core. You can add Heart of Gold or Philostone (Maybe even both)
You'll want to go tanky after that. You can build the Phage into either TF or FM your call depending on the game. FM will give you more tankiness+Better slow and cost less, TF will give you more movement, more damage via Sheen proc (which you'll be hitting all the time thanks to your Q), Crits, more mana (Not really too important but can't hurt), while still getting a slow and some tankiness but will cost you like 1300 more.
Atmogs/AtMallet is pretty common on him too. Wit's End is popular on Tanky DPS right now, Force of Nature is always pretty strong, Frozen Heart is pretty awesome, Randuins is always good, anything that allows you to be a frontline damage soak while still being super disruptive is what you want.
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Whatever...