Well, the armor won't let his arm pop off. Whether that's good or bad, is tough to say. So that leaves the other choices to pick from...
One choice is Bob gets slammed into the ground, this implies Grox has good grip and is heavy enough to not get shaken by it.
Another choice is, the resounding crater in the ground knocks them both down and it's still a bad situation, just in a large crater.
Still another choice, is Grox loses an arm or two from the force of Bob going up.
The last and worst choice is that the cannon is a dud and no one is happy with this result. Bob gets punched and we figure out if "invincible armor" is truly invincible.

While I have a lot of choices, let's see what [@6slyboy6] can come up with that will line up as "god enough".