The speed at which B can fire varies alot. In his fight v.s Sasuke it is neither fast nor unguardable.
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The explosion certainly won't be instant, and Mu's ability to dodge as a sensor is very high. It's entirely likely he'll sense the energy build up, realise what it means, and fly backwards at full speed, using his sensor abilities to dodge the explosion as best he can. His flight can keep up with Onoki, and Onoki is fast enough to intercept a meteorite, so we're not talking about an inconsiderable speed here. He'll have a very decent chance of succeeding. Or, when he senses the energy build up from Bee beginning to charge the attack, just Jinton him in the head. Or when the explosion happens, shield yourself with a Jinton blast which disperses the explosion coming at you, or counteracts it (like when Onoki and Mu's blasts collided).
The fight starts from 100 feet, and since Mu is invisible they won't be able to aim well, Bee's instant suicide (for all the team but him), is their only real chance, and it's not exactly a sure thing. If Mu survives, he just jinton's the badly injured Bee's head off.