The Brainer is a weapon unique to Umbrella Corps, so even players familiar with other third and first person shooters may need some time to get to grips with it.
The Brainer is capable of one-hit killing opponents. As it makes you particularly vulnerable to gunfire, it is best used for surprise attacks.
If two players attempt a lock-on Brainer attack at the same time, they will repel each other. You can turn the tables by heating up your Brainer beforehand, or by performing a counter.
Heating the blade of your Brainer allows it to cut through another Brainer attack, and will win even against a counter. The downside is that heated Brainers emit a distinct sound that can be heard nearby, and which also shows up on the sound radar, giving your position away to opponents.
Also, even a heated Brainer won't help you survive a well-placed shotgun blast, so much like a cold Brainer the best tactic is still to creep up on enemies instead of taking them head on.