Just bury him in sand up to the neck and run the push mower over him. Voilà.

We used to drive two spikes into the top of a block of wood close enough together that you could get the neck in there but so their head can’t slide out in the direction of his body. Then elongate the neck by pulling rearward slightly giving you a bigger target for the axe. You might want to wear gloves. And don’t dawdle.