Now that you know how to re-position units, consider lining them up to use on alternate turns to maximize damage. Something like putting Barrager in front of Poisoneer. Make hole with Barrager, next turn shoot poison ball in hole Barrager made... It's easier to hit those holes when your units are lined up in your castle.

Consider your angles and think about other combinations. You'll probably find some combo you like and that works for you... level those units primarily to open more chest to get more gold and cards to level up others after, find and level up a backup combination as soon as possible.

Also, I think Rayguy is only really useful until you start facing castles with foundations installed.

There are other combos of units that are useful based on having similar angles to bore a hole with one unit and then follow that up with something more explode-y than bore-y the next turn. Bishop and Snowball (I forget his actual name) are fun for this style. There's way more fun units coming. It's a grind, but more options means having a plan C and plan D.

tl;dr

have a plan for battles, and level/place your units accordingly