personally i never seriously used thomas or chuck and not an extreme amount on david. i dont think there is an issue with how much damage a tower does because most of them work really well together. often in time i would build a wall with billies with tecumseh in the middle. the stuns would lock them in place while they died from poison. johnny was my most used damage dealer, his shots are consistently fast with crits at times over 100 damage. i dont think there is any major issue with the game, but i do think there is alot of under utilized things with the towers and most of my struggle was working around terrain. the game does however become a little easy and i never had to replenish my initial 5 cows. i lost only 1 the entire time and that was early on. i rate this at 4/5 for uniqueness and i really had fun beating the game.
You can clear it by saving the game and loading the game