Disregarding the castle defense since it isn't up all the time, if you want a good boost go to the multiplayer settings and set it where it only goes to the smallest maps. As soon as the match starts get out your cleric and set the materials for as many ballista as you can before building them so that the other people can't make Gatling guns. It doesn't really hurt your team any (I'm better with the ballista than the Gatling gun so I know other people are too) and you're ready for the incoming enemies that are going to be there soon if you're not in a really overpowered team. After all of those are built get on one of the ballista beside the gate so you can hit the people that might attack said gate and sit and wait.
If there isn't a really strong mage on the other team you should be able to get at least 5-10 kills before you die (more if you have extra tactical damage on at least one of your items) and as soon as you respawn there are more ballista to hop right on. If they're occupied there should only be that one to build again which should be easy to do since the other 3-5 ballista and the ground fighters are keeping the enemies' attention.
If you have high defense and tactical damage you can get a ton of kills each game with enemies to shoot a good bit of the match time. If you're a low-level player or don't have a lot of extra tactical damage you're still making at least 15 kills a match when the other team is overpowered as f**k.
On a side note if your team goes from castle defense to attacking the castle don't get the notion to join the fight. The battle on small maps can change fast and there is always that one guy that wants to attack the other base alone. Just sit there the whole game and watch them flock to you.