how to kill benjamin with one shot?

there is 2 ways to kill him which one do u mean?

If you are asking about current patch lobotomy, you have to trigger the "warning: intruder detected flushing interior" message then clip outside the earthmover and hit an invisible trigger by the door past where you do the security system fight, this enables the trigger for the boss to spawn while you are outside the earthmover. From there just hit the trigger under the earthmover's chin and it will die. If you are having a hard time seeing I recommend turning up gamma. Both clipping into and out of the earthmover are done by getting on a frozen rocket placed facing down at a 45 degree angle against the wall. Then firing a rocket, jumping and whiplashing all at the same time. The screenshots show which walls you need to clip through.

i keep doing the steps you say, and yet neither of the clips seem to work? am i doing something wrong? is there a specific graphics setting that makes it more consistent? should i try and press one button first? or do i have a ultraskill issue

this run shows how the lineup is meant to work. theres the keyboard overlay too which you can reference as well for inputs