Excessive? Perhaps, but Pariah doesn't believe in half-measures. 

But none of them were really at serious risk. All of the henchmen had non-lethal weapons and the majority of the trainers weren't even using their powers to their full extent. Pray tell, was it ever actually confirmed by Strix that Phantasm was telling the truth about having live rounds? And none of the trainers used more powerful attacks on the weaker members, punching bags like Furious and Changeling could take a lot of damage and bounce back. 

Or if they did like Ember, they knew how to pull their punches. Ember is a pyrokinetic of the highest order, fire is her in a sense. She can turn into fire and she bends it to her will, if she was really trying almost none of her shots would have missed. But she made her blasts slow and obvious to dodge not at all like a flamethrower. And since her fire is a part of her and is an extension of her will she would know if she went too far. If you really thought about it, they all pulled their punches, with their numbers, experience, skills, and powers they could have crippled and maimed everyone before they got anywhere near the fight. Or killed them deader than dead. 

And lastly they're right dab smack in the middle of the Hall of Justice where basically every member knows at least how to deal with basic first aid and battlefield injury with access to world-class medical facilities and even full-blown magical healers. There were no injuries inflicted or even potential injuries that couldn't have been fixed with enough time. So really this is nowhere near what the heroes will face when they come up against the serious villains. It's a taste of the kind of ferocity, skill, and power they will face and seeks to prepare them for it. And as such it is a training mission.