itch.io

Just got to round 25, here's the best strategy I've found:
(first of all, since you will eventually max out at six units, try to avoid all brute units and only go for units that have two of either Athlete, Strong, Pro, and Creative, as this is the best possible combo of traits.)
Wave 1. Get either bodybuilder, skater, or boxer (we are trying to get all athletes as our first three units)
Wave 2. Get another athlete
Wave 3. Get the last athlete you need for the trait
From this point on, try to focus on filling out other traits as you get new units, one at a time. Creative or Pro would be good ones to get next, as this includes the Assassin who is OP (can one-shot zombies once he gets pro upgrade.) Once you get at least 4 units or maybe five, the absolute best strategy is to stay in one of the bottom corners without moving. You want to put your best units on the outside where most of the zombies are attacking from. After you have all six units (assassin, bodybuilder, boxer, skater, blacksmith, & bouncer) and all 4 traits are unlocked, the best units to put on the outside where zombies are attacking from are the Boxer (very fast), Assassin (one-shots), and Bouncer (affected by all of the 4 traits). Use this strat to survive as many waves as possible.
The hardest part of this strategy is surviving waves 5-16 or around there. Once you get the hang of it though, you should breeze through.
Hope this helped :)