This achievement is for beating the game on HardcoreThankfully
if you hit continue after finishing the game (or you can quit out during the Hitler fight)
you will continue back at the Hitler fight again so you can change it to Hardcore and just re-fight Hitler.
To kill Hitler you need to shoot at the wheel under him and it will rotate
Once it rotates a complete turn the laser behind Hitler will shock him
Alternatively you can use the B move and jump into him and it'll kill him quicker
Shoutout to Clarion 85 for this method and also the tip about Hardcore
See in video at 10:55