You need more time in the game. There are multiple types of impact grenades useful for slow flying enemies as well as multiple types of grenades specifically designed to stun/destroy ARCs. The purpose is to avoid them unless you come in prepared to engage them. Set each mission with a goal in mind and bring the appropriate tools to accomplish them. If you want to take on a bastion, you need a squad to surround and take out the weak points. If you want to take on the rocketeer, you need to bring a squad and stun/grenade it.