Trundle is an excellent jungler due to his passive which heals him whenever a unit around him dies as well as his ability to steal damage from a foe and turn it into damage for him.
This build is not meant to specialize in any particular skill set but rather be mixture of Attack Damage as well as making Trundle incredibly beefy.
This build consistently gets me double digit kills with fewer than 5 deaths.
Trundle in the jungle makes for a great teammate during all parts of them game.
When you come out of the jungle, it is your turn to shine in team fights. Trundle's crowd control with Pillar as well as his ability to lessen crowd control with Contaminate makes him a master in the team fight.
Use your skills to slow your opponents while speeding your own movement speed and attack speed. Pick off the squishies on the other team first, as normal. After you have them out of the way, engage the fight when their tank by using Agony. Draining their Armor+Magic Resistance in combination with Madred's is a great way to drain the tank of his HP faster than he can realize what hit him.
Start the game with a cloth armor and 5 health pots. This should be more than enough to get you through the first phase of jungling. After you go back once pick up a long sword (and if you have enough go ahead and get Madred's Razors.
After Razors the build varies. If you are still in the jungling phase, then it is okay to go ahead and pick up your Pickaxe. If the game has swung to ganking and team fights already, then pick up your Treads as you don't want to be left in the dust.
After your Treads finish off your Bloodrazors. This in combination with you Agony is an absolute tank drainer mid to late game.
Next pick up Frozen Mallet for added health/damage as well as its slowing ability.
Spirit Visage is a great item for Trundle as it boosts his passive by 20%. When in a team fight, if there are minions around, it is a great way to gain back valuable health as you gain a percentage of their health when they are killed.
Next pick up a Sunfire Cape as it will make you all the beefier with its added HP/Armor.
Finally, I go with Atma's. I find it is an undervalued item. It gives Trundle extra Armor+Crit Strike which for a beefy fighter is always crucial. As well as providing him with increased AD for a percentage of your HP.
Start with the Golem on your side of them map. With rapid bite, 1 health pot, and smite this is a relatively easy foe to slay. Next move on to the Wraiths and Wolves. By this point your smite should be up again. In combination with the attack speed boost of contaminate, rabid bite, and smite you can take this creep. (Sometimes you may appear to be cutting it rather close.) Next get the 2 large creeps at the bottom of the map and continue in this pattern. You should be able to get the Dragon at level 6 with relative ease especially if you use your ultimate to squish him up.
Once you get to level 4 and posses pillar it is a good idea to start heading to the mid or the lane in which a teammate is soloing to go for ganks as your slow is very powerful (some would say OP) and makes ganking very easy.
Trundle has a very unique style under this build. He is not a heavy DPSer nor is he meant to be a straight tank. This build allows you to do a hearty amount of damage as well as take even more in return.
Where Trundle really shines under this build is in team fights with his ability to slow, control crowd control, and Agony's ability to drain Armor+Magic Resistance from your target. Add that together with Madred's damage passive and Frozen Mallet's slow and you have yourself an absolute team fight game changer.