Step 1
Remove all sound but the firefly sound/ping.

Step 2
Listen for the ping.

Step 3
At once press W. (or as soon as possible if a firefly is already on the "screen)"
................................................
Thats it.

As
A firefly hit with the Shuriken Vortex (W) multiplies the firefly or you get a multiplier.

(rare cases you get a storm)
"Ignore them"

You gain way more by focusing on the ping
if you are lucky you get a 2nd bonus multiplier

And example 5 x 4 = 20 x the W multipler on the firefly

Or if super lucky 4 or 5 x 51 x the W multiplier on the firefly.

= You break the rest of the math for the game.

..................
And of course keep the "normal" speed/coin multiplier up.

But unlock the Shuriken Vortex (W) and the math will "break" with the ping/press W tactic.