The waves of devourers just kept on coming amd coming, each one stronger than the last.
Ordinarily, the fight was meant to be much harder on the mages deployed to fight. However, the surrounding mages barely had to do anything. That was because Gideon was basically carrying this entire battle on his head.
The moment the next wave arrived, Gideon would infect the devourers with his parasite poison and turn them against each other. Other times he'd use the blood explosion poison and create chain reactions that destroyed enemy lines.
At some point, the devourers finally recognized that Gideon was the biggest threats here, so they momentarily stopped their assault on the fleet and focused on him.