As the one who set up the fight, Roy felt a responsibility to make justice for both sides and take care of the big differences.
For that reason, the moment that the imprisoned guy arrived at the door, he quickly cast a few Water Balls of magic and hurled them towards the formation in front.
Since the attack came from behind, and everyone was focused on the fight that was about to break out in front of them, they didn't really realize it was his doing.
Of course, normal Water Balls wouldn't make much of a difference, at best it would break the guys ahead balance and attention, helping these imprisoners have the upper hand by attacking first.
Unfortunately, Roy wasn't such a simple person that would make such a weak move, as each of these Water Balls had in its center a much smaller Acidic Ball.
While the difference in size was more than obvious, that much was enough to actually melt the shields and weapons of the guards in front.
