Ask a question, and I can answer it for you. Every body just calm down...
Any Attack/Raid/Mission can trigger a boss. Once a boss is triggered, you have a limited amount of time to defeat him or else he will get away. In the end, you can win a free indestructible unit. Each boss you defeat, the next one will be even harder. So it starts easy, then gets extreamly hard! I am at like level 34 boss and it takes like an hour to defeat with the free option.
I recommend using the free option from the start and never use gold. The mega attack doesn't do much damage in the harder levels.