Want the hard badge? focus on Damage, it's the best approach, all the UP for damage until it's maxed. Then i would turn to rate of fire. Also, when a new wave is coming, try not to get sandwiched between the spawn points, in later levels it could be deadly... (i should know, i got effed on lvl 29 1/2)...
Also loving how your "base" gets dragged torwards the enemy when you're trying to run, funny, as that doesn't happen to my enemies. 1/5 out of sheer frustration.
I won using 1 of every king of tower in each elevator, except light MGs, it's actually quite easy that way, get every elevator the moment you're allowed to, then get one of each gun. Once you have one of each, upgrade. I almost didn't juggle doing this. At the end it was an auto-win.
Lvl 3 and the flamethrowers, what a damn annoying lvl, you have to place your stupid flamethrowers then hope you luck-out and they don't get killed, you manage to place 3 more, then comes the mixed wave and all is lost...
 "City Siege 3: Jungle Siege"){kind=icon}
 "Dibbles 2: Winter Woes"){kind=icon}
 "Theme Hotel"){kind=icon}
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 "Bunny Charm"){kind=icon}
 "Star Claws"){kind=icon}
 "Sota"){kind=icon}
 "Survival Lab"){kind=icon}
 "Reincarnation: The Final Happy Hour"){kind=icon}
 "Nick Toldy and the Legend of Dragon Peninsula"){kind=icon}
 "Zombie Tower Defense: Reborn"){kind=icon}
 "Pixel Legions"){kind=icon}
 "Spectromancer: Gamer's Pack"){kind=icon}
 "The Next Floor"){kind=icon}
 "Mushroom Revolution"){kind=icon}
 "Use Boxmen"){kind=icon}
 "Desert Moon"){kind=icon}
 "Tangerine Panic!"){kind=icon}
Glad I could eek out that extra point!