Go Mecha Ball

Midas Wrath is an ability unlocked for 250 blue coins. It was among the last abilities I unlocked. Its description reads, "Trade cash for kills, increases your gun damage but each shot costs a coin for a short duration."

To unlock this achievement, you must shoot 100+ shots during one activation period of Midas Wrath. You'll need a gun with a high rate of fire and bullet capacity over 100. Fortunately, character Sprout's default weapon is the Rapido, which fires at max speed and holds 200 bullets.

Play as Sprout, then acquire Midas Wrath, 100+ coins, and 100+ bullets. Activate Midas Wrath, and then hold the trigger to fire 100+ shots. You can watch your coin total go down to track your progress. It doesn't matter if you hit anything, so you can do this after everything is dead to avoid possible interference.

If you want to focus specifically on acquiring Midas Wrath…

The screen offering three random upgrade / ability options can essentially be rerolled by quitting and reloading the game. Choose Resume at the menu to load in at the end of the preceding level, and immediately advance to the random upgrade / ability screen again with new options. Repeat until you get Midas Wrath (or whatever you prefer.)