Now think about this logically. What use would it be to have a troll champion that can't troll allies in any way?
On a more serious note, Trundle is intended to be a spectacular duelist, particularly against fellow melee bruisers. He can, however, be easily poked (if you're consistent about it) and kited, and focus fire in a teamfight can take him down quickly. He excels when he is able to stay in a fight for longer than a few seconds.
Do not sit and trade autos with Trundle if at all possible. Refuse to fight on his terms (specifically, don't stay in his Frozen Kingdom) and try to make him waste his Pillar; it's not cheap, and the long cooldown leaves him vulnerable.
In a teamfight, Trundle wants to reach and sit on the enemy marksman, or any other AD-focused damage-dealer. Often he'll ult the tank along the way to sustain himself. Don't let him reach his target; stun him or knock him back, blow up his teammates, and finish him off. Don't half-heartedly burst him and then continue trying to DPS him to death while his ult heals him back up; it's gotta be all or nothing, kill him immediately or leave him for last.
Trundle is similar to Udyr, Volibear, Olaf, etc.; his engagements are incredibly predictable due to the lack of any dash abilities. He has little choice but to run directly at you to try and beat your face in. One good slow can ruin his day.