Don’t fight Trundle if he is ahead. His skirmishing power is incredibly strong especially if he has his Ultimate R available.
Trundle cannot really gank if he doesn’t have any strong lanes- so he will look to power farm instead. Try to gank as much as possible so you can get your allies ahead.
Trundle has good objective control and can solo them with ease. Keep them warded at all times to make it harder for him to secure them.
When Trundle activates his Subjugate R, back away immediately. He will become much tankier and your tanks will be weak. Re-engage when his Subjugate R has cooled off.
As Trundle will be split pushing a lot in the later stages of the game, try to team fight when he is not able to be there. If you spot him isolated and away from his team, try and force a team fight with his allies.
As soon as Trundle engages in a team fight, try to lock him down with CC. While he is a tank, he is very vulnerable to hard crowd control.
Once Trundle hits level 3 he can start to gank as his E is the main tool he needs to get ganks off.
If you are a tank, Trundles level 6 all-in can be deadly as it reduces your defensive stats to a crisp. When trading, be careful around the level 6 mark as he may look for an all-in.
Depending on what build path the Trundle has gone for, he will peak at different times. Keep this in mind and always look to see what item’s he has purchased and play accordingly.