Board Thread:Dragon City Gameplay Questions & Answers/@comment-13428995-20130808021209/@comment-11535585-20130808050340

You can complete the level after the island rotates. You will get a different opponent (not the 20 eggs one).

The only requirement to complete the level is to beat 7 opponents, not to beat them all. The difference is, you can leave 1 or 2 unbeatable opponents and still move on to the next level.

This is one of the nice feature from DC :)

Btw I don't see this info in Combat page, so I will add it.