According to Takeshi's response, that wasn't a normal Beast. The previous attacks by Ozymandias and Takeshi could have killed a 'normal' Beast easily: this one, instead, endured all of those onslaughts and was having the upper hand at the moment... this, because of the Beast's last attack. Everyone was 'sent flying' by that pulse of energy, and while Dexter and Ozymandias were recovering, Takeshi was pinned by the Beast, which was ready to get its revenge on the mage.
It was just a matter of seconds before the attack was delivered.
"I can't let this happen... again." said Dexter, as he pulled out his pistol, trying to land some shots on the Beast's head as he started to run towards it, in a desperate attempt to save Takeshi. For some reason, his blue eye was glowing more, as his objective wasn't something like hurting the Beast, but only make it so the Beast would let go the guy.
He was hoping to shift the Beast's attention away from his friend, who was visibly more tired than him, and let him escape... but he didn't really achieve that result, as he appeared to be in the position of the one that would save someone else at all costs, saying things like "Want to kill him? Kill me first". (And of course, killing Dexter before killing Takeshi was an easy task anyways.)
"I'll need some help!" Dexter shouted, trying to get the attention of Ozymandias. "After this, we run..."
He wasn't even sure if that could have worked; he didn't plan to do what he'd just done, it was all a chaotic and desperate sequence of actions.