It took me a handful of tries on SCH without abusing barrage but it can be done.

You have to stick towards the center, open with LB if you can to get damage buff early. Any time they stray to take out adds, use Engage to get them back, if the boss moves with them, disengage and re-engage them when he is back in position.

The key is to get them to just focus down the boss as much as possible and stay away from the edges so the larger tonberries don't ruin your day.

If you haven't already, I would suggest also getting a few ranks into the AI settings for the damage bonuses you get from them, don't jump straight into WP.