Heh, I got bored so I went back and beat Cave of Trials 2 and never hit the enemy castle with anything but mermaids from all the way at my castle. I used Pheonix Strike to clear the path and the mermaid's attacks got all the way there. Took a while, but it was an interesing way to do it. Most of my units never got halfway across the field.
 "Platform Racing 2"){kind=icon}
 "Epic War 3"){kind=icon}