For ranged units & mechs CD is definately the way to go but you can remove layers.

These are a few months old but it gives you a general idea.

http://www.evonyurl.com/2rgax0
http://www.evonyurl.com/xq3xcp

You can do the same by clearing all the layers when facing a mech wave. I do, however recommend if you do that load your city up with scouts. You'll need 15k ballista at least to repel a mech wave.

This was my first mech wave defense. Hopefully you'll spot the mistakes I made.

http://www.evonyurl.com/pkcevb