Trundle excels at fighting within his Frozen Domain. Try to draw enemies onto it.
Use Subjugate to soften a powerful enemy tank or to create a target for your team to focus fire.
Chomp is good for lowering the physical damage of enemies; try to focus it on enemy physical damage dealers.
Trundle is very powerful at location-based combat. Try to draw him off of his Frozen Domain.
Make sure to get out of his Pillar of Ice as quickly as possible, as it significantly slows you down.