Yes, if you destroy you brainstem it will kill you guaranteed, this is because your brainstem is what allows your brain to send signals to your body telling it to pump blood or breath, so if it's destroyed then even if you don't die immediately you'll likely die of suffocation or heart failure. it's the best option if you want to ensure that you will die. However the only real option is probably a firearm, with a firearm, I have identified 3 locations you can use on your head.
1. Nose, You can place you firearm directly onto your nose just about a few centimetres/inches below where your eyes meet.
2. Ear, You can aim just above your hear hole, and you would hit your cerebellum which would kill you instantly, or you can aim above you ear hole and a little to the right and you would hit directly your brainstem.
3. Mouth, You can place your barrel into your mouth and angle up placing the bottom of the barrel at you lower teeth, and the end of you barrel towards the bump at the back of the roof of your mouth.
if you don't have access to a firearm, I would reconsider, or try and find another method. Hope this helps you.