Between classes at the moment, so I can't give you the specific code for a modlet, but you need to change the EntityDamage of the player fists as defined in line 5920 of items.xml from the default base_set value of 5 to something egregious like 5000 (like the dev tool Hammer of the Gods). I know if you pop a zombie in the editor with that hammer the results are often hilarious.