Hello, everyone!

By now you've realized that the baseball bat and frag grenade upgrades can help you wipe out a map pretty well, but there are still some risks involved. My method will allow you to stand still (going afk even) and finish a map without any issues. This is more of a late-game build (since you'll need some unlocked weapons/abilities), but it's a small list of necessary items. Here's what you'll need for this to work, with notes on how to obtain those items:

• Amaterasu - Ascended form of Kusarigama (unlocked by killing 10,000 zombies) and Prolongation Formula (unlocked by holding out for 10 minutes as a Biotech). This is the most important item in this build, so even if the other pieces don't work out as well, make sure you accomplish this one. Amaterasu has a built-in knock-back function that keeps everything outside (except the shield zombies - but they're no concern).
  
• Biodestructor (unlocked by holding out for 7 minutes as a survivor) - This item is critical as it creates a zone of safety around you. It also benefits from Strength Drain (weakness and poison-type damage) It's also my favorite item to help you blast through early zombie slaying.
  
• Creeping Death - ascended form of Toxic Grenade (unlocked by reaching raid level 20 as a Biotech) and Strength Drain (unlocked by killing 1,700 zombies as a Biotech). While Toxic Grenade creates random pools of poison, Creeping Death centers the damaging field around your merc - which is going to keep you safe (and kill zombies).

These items are your core, and the rest is really up to personal opinion (even the weapon and armor your merc uses, because that doesn't matter in the least). However.... the following items are what I like to go with, as I think they compliment things:

    
• Gigantism (unlocked by killing 1,000 zombies as a Pyro) - Makes your zones much bigger, which keeps zombies further away (and kills them before they get to you).
      
• Mag Pouch (unlocked by killing 1,000 zombies as a Scavenger) - it's 2 additional projectiles, which may not seem like much... but that also means 2 more steel balls of death to destroy zombies.
      
• Shocker (unlocked by using Large Shock Grenade 5 times) - when fully upgraded, it'll shock zombies in a clock-like order (quite fast). Any methods to keep zombies from moving away from your blender of death is a win in my book.

Once you get those core items "built," you can literally stand still and let the zombies get wrecked as they try to eat you. Before I let you get to it, a few small warnings (which may include spoilers, so I'm sorry if that's the case):

    
1. Amaterasu's knock-back functions best when you're standing still. The damage piece will work fine when you're moving around, but zombies will get inside the field if you're moving. I know it's tempting to go gather that exp (or chest), but make sure you can handle it.
       
2. Just before 25 minutes has passed, a boss/elite will spawn that has a charge/rush attack (you'll see the red rectangle to denote the area it's going to run in). It's truly no concern, as your damage field will dispose of it very quickly (and you'll have full health anyway), but I didn't want you caught off guard).
       
3. I had mentioned it earlier, but the only enemies that will walk through your Amaterasu's knock-back field are the purple shield zombies. You should see them enter the field and die quickly, so they're not a terrible concern (unless you're moving... which you should not be doing).
   
4. After the 25-minute mark, the big lab coat-wearing zombies will spawn. They will not be able to enter your field, so don't be concerned! Same goes with the big pumpkin head-looking zombies that spawn around the 28-minute mark.

Ultimately, I use this build when I have to complete a lengthy task (i.e., survive 29 minutes, etc.) and don't want to run around. Good luck!