History of Catching Waves

Catching Waves is a 5-point triumph: "Complete the 5th wave set in Onslaught on Legend difficulty."

How to complete

Play Onslaught (the Onslaught: Playlist) on Legend difficulty and clear through the fifth set of waves. Wave sets are the groups of waves ending in a boss; plan for a Fireteam with strong Champion handling since Legend is the higher difficulty. I could not open a guide that details wave-by-wave tactics, so no further tips are confirmed.

Rewards

5 triumph points. A tweet from Destiny Emblem Collector (search result snippet; page not readable) says it unlocks the "Reborn BRAVE" emblem, introduced with Into the Light. Related: Onslaught Mastery.