Board Thread:Dragon City Gameplay Questions & Answers/@comment-24932909-20140530105818/@comment-24964189-20140602111641

It doesn't matter so much that your Pure Sea can't critically hit Terra, as long as it outlasts the opposition. If you can get ranking stars via Combat World (there is a current glitch where the ranking doesn't permanently save) then it effectively makes your dragon higher in level - I think 3 gold stars is similar to an increase of 5 levels.