Board Thread:Questions and Answers/@comment-2199535-20150818233035/@comment-25730832-20150819000028

You can reduce the boss meter by just attack the boss. Hence, chipping down its hp bit by bit until the end the bar got empty.

HOWEVER! That doesn't mean you clear the map. To clear the map, you'll still NEED TO SINK THE BOSS 1 LAST TIME in order to clear the map. This allows you to keep trying again and again to reduce the boss meter bit by bit before clearing it up once and for all in the end...it's just...tedious.