Braniac has flying saucers with impenetrable force fields, shrinking rays that put entire cities in glass casings, and tons of nanomachines. If he can't control the sentinels, he will definitely create percolating nanomachines that seap into the sentinals, which will then dissolve everything inside like antimatter. Not to mention, Braniac goes everywhere. He'd relay himself via satellite, send himself into whatever base is giving power to the sentinels, and cut off the signal. That's just the way Braniac rolls man. He's a pimp.