I bought this game 2 days ago so rest assured this actually works even for someone who is an inexperienced as me (I got it first try).

Just want to add my experience in case anyone else is trying this in the future. It takes a couple rounds getting used to playing such a difficulty solo, but with a low population regularly playing at this difficulty it will likely save you a great deal of time. Defeating him is easy as you can retreat to heal then use the Pulverizer to take off armor and health. If you are lucky you get the Abomination who has to chase you. Second the boss is randomly selected and may present you with one that you won't be able to defeat with melee weapons. First if you get all of your large enemies at the same time it will be difficult but possible with the Pulverizer. The rest is getting lucky in a couple of things.

The Pulverizer's secondary fire is an explosive charge that deals with Flesh Pounds in two to three hits while everything else can be dealt with by the Bone Crusher over time. During the game equip yourself with a Pulverizer and Bone Crusher. This provides damage resistance and healing on kill and over time. With a level 20+ berserker use the skills Skirmisher, Vampire, Resistance and Massacre. Each of these will likely require some dancing on the stairs to dodge their attacks. The only exceptions will be the Scrakes, Flesh Pounds or Husks. As suggested in others, stand in the entrance to the guard tower facing out and melee anything that comes in range. Set the game up for four waves on Hell on Earth Difficulty on Prison map. This achievement can be completed solo with a little patience and luck.

Invincible scrake killing floor 2