Groot is an interesting puzzle. Half of his kit wants to stack counters and the other half wants to spend them resulting in a bit of a dilemma. After some testing I think burning through the counters is the answer. You can spend several turns stacking for a big event just to have everything ruined by one bad encounter card.
The ideal scenario would be blocking for a partner, getting hit directly, and ultimately ending up around 2 HP (5 or less if you have endurance). This gives you an excuse to stay down for a couple turns to stack counters. When you flip up you’ll have 1 turn to play your events with your freshly stacked counters before repeating the process of spending them. Obviously this strategy really only works in 2-4 player and really needs to be paired with Justice to help enable your semi-frequent flipping.