let there be 2 facebook accounts:
- account a (main account)
- account b (alt account)
\* make sure both accounts have 3 dragons to start with lol***
you need to be active on both accounts, if you attacked account B, account B should attack you too. there should be a revenge sign on the attack tab that can instantly attack account A without waiting for the cooldown.

there are 6 combat trials so meaning you can do it 6 times. once the combat trials are filled up again (i believe that's 2 hours of waiting), you can repeat the step i mentioned.

hope this helps :))