in the end I started using my auto-clicker I use for the rock mining games on here. It makes the spamming and grinding easier. In game design feedback i would say; great skeleton graphics, but the castles could change - graphics, upgrades, terrain etc and the upgrades are too expensive. If the basic arrow is upgraded halfway it should one-shot the lower half of the mobs not just the weakest.
easy way to get the jump 6, is to get to the tank. drive it into oncoming traffic and let them pile up in front of you. When you have 6 in a line, get out and jump on to each.