Dexter was able to dodge the Beast's tail, like Takeshi did, thanks to his improved agility when transformed. Seeing the beast laying on the ground and Takeshi's ready stance, the blonde guy realized what were his friend's intentions. It seemed to be the time for the final blow, and Takeshi was the one who wanted to inflict it more than anyone else.
Dexter didn't have much to do; there were Takeshi and Ozymandias with him. The former killed that other Beast, when Dexter was still a human with no contracts and couldn't do anything but hiding in a building; the latter killed the other Beast which showed up before that week of training... that was concluded with another encounter with a Beast: the one they were fighting at the moment.
Both of his friends were more experienced than him, who had just begun his new life as a magical person. Because of this, Dexter had always avoided to enter direct combat with the Beasts, and let other people do the job, like if he was just a spectator... he wouldn't get himself in danger, sure, but his friends were doing everything.
"Alright, let's do this. I'll just make Takeshi's job easier." the blonde guy thought, as Takeshi was still gathering energy. He just jumped towards the Beast, still staying at a reasonable distance, and tried to land a shot with his shotgun. He wasn't trying to kill the Beast himself - Takeshi was basically coming in for the kill right after him - but only to make it unable to react at anything his friends could have done.
He ran further away from the Beast after hitting it, following Takeshi with his eyes.