He should quickly blanket pardon himself so the Republicans push to codify that a president cannot pardon himself.
Then he should arrest and seize the property of all members of the opposition party under some BS national security claim. The Supreme Court has already granted immunity for this sort of actions by presidents so no need for another pardon.
I have no experience nor done any research into this topic but I would think that a gun to the temple is easier to mess up resulting in either a slow death or a non-lethal shot with a life altering disability or disfigurment.
Gun in the mouth is more guaranteed a kill shot. Under the chin would probably be even more guaranteed. Just be sure to aim up and not back.