Brotato

Info about this Bro:
This Brotato does more damage the more money you have, so we want to keep lots of money to do more damage, but items cost 50% more, so we don't want to spend money.

Here is what I did to get this achievement:
1.) Picked any weapon as starter weapon, run around and complete the first wave.
2.) Sell weapon and get either slingshot or shuriken, something that bounces to help clear packs.
3.) It's ok to spend some money early on we want to get 6 weapons (slingshot/shuriken), you only need grey ones as soon as possible.
4.) Once you have 6 weapons, just focus on level up stats +speed to move quicker to grab money and normal damage stats like attack speed or range weapon damage.
5.) Just keep playing until the end, the more money you get the more damage you do and will destroy the packs around wave 10 or so.

++You can also get the money achievement doing this one as well if you didn't already do it.

Hope this helps! I know this works for me so I wanted to share.

-Dalfurious.