Gulgrim is essentially swinging wildly after being unleashed, still not particularly able to aim after the flash from Jharm, so he's certainly not all that concerned with whether his strike might interfere with anyone else's plans. The swipe is an opening barrage from a rather pent up Gulgrim, so it's likely relatively forceful, though it's only going to get worse the longer the fight goes on. As to whether it would come before or after? He basically started swinging as soon as he registered Soul Reaver's okay, and if Soul was finishing his spell as he said that, he's probably only starting the swing just afterward. It might break the cocoon, but unless it's completely rooted to the ground it might just be sent flying by the strike (and even if it were completely rooted, the ground is riveted together paneling on an Orkish krooza, there's not a lot saying that section of the floor wouldn't give way to go with the cocoon if it's particularly durable)