That post-credits thing how to achieve?

Watch the credit ending of dungeon 3-10 until the end, this is the level where you and the Captain defeated the Warlock, after you and the Captain entered the portal and the level ends, you will see the ending, where there are lots of guards patrolling the Castle entrance, until you reached the part where the main screen of Magic Rampage is, there will be a portal with a to be continued text above... that's the ending credit for 3-10.