Very simple. Go to the towers of the horde ( with catapult is the most ez way )
jump on the tower. Start attacking a cannon wich is at full health and profit!
you get weapon skills of those cannons for some reason. And @ horde towers you can stand behind a kind of pillar and attack the back cannons wich noone ever uses. And if you dont kill the cannons horde players are inn they useally ignore you so you can continue to skill your weapon
Didnt test this for horde attacking alliance cannons
just my 2 cents