I was considering using an Amoonguss as my grass type, but he honestly seems very underwhelming. Maybe a Rotom-C?
Amoonguss can give trouble to some teams although Goodra and Mega Venusaur are complete stops to it. Rotom-Cut could work but it's important to think of what the Pokemon actually does in the core instead of just making sure it's a Grass type. Without that Steel or Poison typing, it can't really take Play Rough from Azumarill, and I think it's just generally not as useful as Rotom-W due to having so many more weaknesses.
Honestly, I'd see if you could give Ferrothorn one more shot, what kind of sets have you been using on it and what has it been losing to? I am kinda interested in using it myself, it's a favorite of mine in Doubles but I've never really used it in Singles.