Not only was Satoshi covering for everyone on the offensive side of things, but he was also taking care of the entire supporting role while defending everyone just in case they had any openings. It took a toll on his energy expenditure, of course, but Satoshi decided to do what he could before the superheroes arrive. If he didn't step up to the plate, there was no way that the four of them would be able to hold back the entire army of dragon-like beasts. Another reason why Satoshi had to overextend was because of Shiroi's dad and his mom. The two of them could only hold the dragon-like beasts out because their superpowers are meant to take on the supporting role.
If it were only one dragon-like beast, Satoshi would've easily taken care of it. But he still had to support everyone on the team so it was hard for him to even take on one. Fortunately, Gabriel and Ice Empress were doing easy work with the beasts. They didn't need any back up at all.