Spoilers, obviously.
Basically Brixton's eyes scan and body read people in his field of vision feeding him info from the probability of someone beginning to attack him (the first MI16 agent he disarmed and took out IIRC) to exact figures about the strength and speed of attacks already moving to him (from Hobbs and Shaw) so that he could react and counter them optimally.
Even without that and with the element of surprise they couldn't physically match him alone. Shaw tried to choke him from behind with a fence cutter tool only for Brixton to break it and shake him off. He got up from Hobbs swinging a ripped helicopter door at the back of his head with two hands while Brixton was distracted attacking Shaw, even if it did hurt him (Hobbs is the Hulk to Brixton's Superman in this movie) but that was the moment they realized their only shot at beating him was teaming up. It took several more attacks before his defense system started to malfunction.