When trying to stop a Brainer lock on (PS4: R2 button, Steam: RT), you could try shooting,
but there's also another very effective technique.
If you perform a frontal melee attack (PS4: R3 button, Steam: right stick button) at just the right time,
you'll perform a counter which not only cancels the opponent's attack, but will one hit kill them as well.
If you're in a position where you're expecting a Brainer lock on,
try spamming counters if you're unsure about the timing.
Remember this won't work if the opponent attacks you from behind, or if they have heat charged their Brainer.
Keep your ears open as you'll instantly recognize heated Brainers by the noise they make.
The July 7 and 8 updates have also nerfed the Brainers somewhat
so combat should be a little less frenetic than before.