This achievement might be possible in Hero Mode, but I waited to knock this one out until I had completed the 3 Hero Mode Achievements:
- Hero Mode
- Hero Mode +
- Hero Mode ++
After this, I had a massive surplus of scrap and was able to recruit and outfit every one of my new characters with decent equipment. Additionally, all the characters were Level 45. For the challenge, here is what I did:
1) Place my 6 fully leveled defenders in the bottom left leg (focus there for the Berserker, Priest, Ice Mage, Knight, and Dragon) with my Ranger towards the middle
2) After fully leveling them, I began placing other groups of units at the other appendages and leveling them completely.
3) With everything leveled and placed I just let the challenge run until I had surpassed Wave 100. Most enemies couldn't get beyond the 3 group even when I was up above 100.
The most annoying part of this achievement was just having it run long enough. The game really was laggy as I hit the Wave 70 region. Once you have your defense set, you might be able to just walk away for a bit and let it run to completion. I actually was at about Wave 110 when I recalled all my defenses and allowed myself to fail to end the challenge and earn the achievement. Didn't have to, but I was ready to be done at that point.