I just bought energy to secure a monster Hunter and used to to fight and someone still took the monster. But I still lost the 5k energy and gems all together when I didn't even attack it. The cost of thing in gems are way too high and getting the energy taken for nothing, is a ridiculous trade.
The energy is used when you send your hero out for the attack. If you're going to attack a monster then you should make sure that it's not already being pursued by another player. It's not up the the game developers to strategize for you <3
Personal Might: 10mil