His insta heal is big, but only when ally is in his circle. Firstly you and "your allies" should pay attention to "his allies" and them occupy with something. Then when Grover is left alone it's your chance to challenge him on solo fight.
If a Grover is by himself, this can give any champion with decent mobility a chance to close the distance between them and Grover. The closer you get, the more damage you do and the less damage Grover does. Don’t forget about the vine though!