Save your explosive flask for Skye. It will slow her down to allow for more precise follow-up hits and will reveal her if you have the "Catalyst" legendary selected.
Using a shock pulse focused loadout, Grohk can frequently slow down and damage groups and even hurt fragile champions behind cover. Use hard shields (Barik, Fernando, Makoa) frequently to protect your team.
Ruckus is a good champion to use "Reversal" on; it might even outright kill him when he's using his ultimate.
Try to get in front of him quickly when he's already firing on your teammates, but be sure to have an equally fast escape!
While Drogoz is flying, skirt his attacks with "Weightless". The "Aerobatics" legendary works well here. You can attack him back once he has landed, as he moves very slowly on foot.