Bill released his wind aura and cast the vacuum cutter skill. It wasn't as strong as when he used it against the Beetle King while his mutant gene was still active but it was still quite strong for what Bill wanted to achieve.
The unstable fireball was already on the point of breaking down and exploding midair, before even reaching the Terra Beetles, but the vacuum cutter blade reached it just in the nick of time.
Boosted by the wind aura, the fireball quickly broke down and covered the blade completely, taking on the shape of the vacuum cutter blade itself.
It was very similar to Mako's Flame Wheel skill when he used it only have a way to create a blade of fire instead of the whole wheel, but it was stronger as the fire was boosted by the wind.
Since Mako's Flame Wheel skill had gone up in levels several times, it was still the superior skill as compared to the Fire Vacuum Cutter which only used a level 1 fire aura, so the best it could be compared to is a Level 2 Flame Wheel.