Whirlpools are nice. They pay back the mana used to cast the spell, and it can be used almost anywhere. Once you build up enough castles, you'll be able to cast whirlpools as many times as you want. Also, drown enemy heroes. It is easier to drown them than to fight them.
"General, they can destroy 2 EMP tanks even if we send them together! Shouldn't we send more?" "No! Send 2 at a time, then give them a time to recover, then send another 2! Don't ever send more than 2 at a time!"