So how will you beat it? Ice, ice baby =) Freeze those zombies and use either the blaster or the laser from your engineer in combination with your abilities to kill them endlessly. Of course you should already have spent your teampoints to get all upgrades and unlocked at least some of the main perks, that we want to use:

• (for boss buff) Coffee cup
  
• (for boss buff) Sugar rush
  
• (for chest buff) Item chest
  
• (for electric damage) Electric personality
  
• (for electric damage) Icon of tempest
  
• (for electric damage) Jacobs ladder
  
• (for elemental damage) Detectives pipe
  
• (for elemental damage) Zugzwang hypergaster xD
  
• (for ice damage) Frozen Heart
  
• (for ice damage) Icon of stillness
  
• (for ice damage) Special snowflake
  
• (for SOS signal buff) Ring of power

If you have trouble to get it on the endless mode, just pick the boss rush mode to benefit even more from the boss bufffs

Team:

1. Engineer
   
2. Medic
   
3. Hunter

Weapons to focus on:

1. Engineer - Blaster or Laser
   
2. Hunter - Freezing arrows
   
3. Medic - Freezing flasks

You want to level up your weapons fast, also the shield and the stim pack. Focus on the perks above, but remember to use your rerolls at the beginning, to get your correct team members. I always try to get DPS for level ups, so damage and crit+ for weapon and perks always goes above heal or luck. You can also get some armor and life points, if you're getting too squishy. And don't forget to level up your revive from the medic, cause you never know, when you'll need it.

Try to banish not much at the beginning, as we go for a long run, we will use all abilities and nearly all perks. But you can banish some with chem or fire bonus, as we won't use it. If you will go for 100+, try to use the banish options on stats, where you reached the cap limit. With this build I reached level 222, and could even played longer. So I hope it will help you too to get the achievement.