Orcs Must Die! 3

This achievement is one that will come over time and long before you complete this game so it isn't necessary to grind for if you plan to 100% it. Outside of that, here are a couple of tips for those who do want to rush this achievement.

Combo, combo, combo, the higher your combo the more coin you will get by exponential amount. Try to always keep your combo going and as high as you can get it. This meter goes down really quickly if you aren't getting kills often. Every kill counts towards your combo. Whether it's a trap kill, your kill, or one of your friend's kills if you're playing co-op.

Endless mode or higher tier maps. The more enemies the better as you can get higher combos. The best map I have found so far for this is Coastal Hallways endless as it's an easy map (Probably the one you will use for surviving 25 waves) and has tons of enemies to get lots and lots of coin.

Play solo, the more players in your lobby the more ways the coins are being split. In solo you get the same amount of enemies but you get all of the coin as there is no one else to split it with.
Solo = 100%
Duo = 50%
Trio = 33%

Good luck and have fun.