We he uses his ultimate, try to separate him from the rest of the team

Encountering Grover in long range open spaces - don't PEEK around the corner. Good Grover players throw axes preemptively and you can take a lot of damage. Use short sprints between cover. This way it's harder to predict where to aim.

Don't embrace a fight when he is using his ultimate. It heals a lot of life points of all nearby enemies