How do you do it?

Level 6 and it's good game 90% of the time. The other 10% his team doesn't commit. So how do you counter him in Treeline?

kite him

That's easy enough to say, but not so easy to do. We're talking about Treeline here. The map is small, you have 2 other teammates in close proximity at all times (except that 10%), and he has a gap closer.

Lock him down with CC post-ultimate, or just buy a warden's mail > Randuin's.

A Randuin's Active turns Trynd into a joke.

Malphite, Nunu, Lee Sin.

Ignite.

kite, every time I play ranked and they have a trynd we pick zyra with exhaust. she puts down a couple plants, ult and exhaust he cant do anything while you kill his team then come back for him.

Yes, Hard CC champs do pose somewhat of a threat, but you are also dealing with 2 other champions. Unless your entire team has hard CC, I don't see it being that big of a problem. If you pick Zyra, for example, you might be able to stop Tryndamere for a brief moment, but that's not going to kill him. Hexdrinker usually prevents an AP's burst and with his Ult and Q he can pretty much be full health again immediately. Dealing with a Carry? Randuin's Omen, like Anders said. He can pretty much counter your counters.