What does the last achievement of Local Legend Crab mean?

"Shoot down the Crab Tsar before it executes its Almighty Bombardment for the 2nd time"

So I need to stall this time, and not finish it quickly? I have to see the boss do the 2 rings of spawns a second time, which sounds easy? Knocking him down seems trivial with Nahida, it always happens?