The only reason why trundle is a viable fighter is because he is very very busted.
slippery champion with high damage, zoning power, and an ultimate that makes him unkillable and makes your tank a squishy.
To be clear, i do not believe he is in need of a rework, but his ult/passive definitely need some tuning. He's wayyyyy too tanky.
counters him, but I don't see how personally, I mean she's a definite counter in a beauty pageant though.