First up, to obtain the Randomizer you’ll need to head down to the bottom of the Modifiers menu, done by pressing
in the level select screen or by toggling Custom Mode in the Arenas (
), going to a challenge, and pressing
to open the menu. You’ll need a pretty large sum of 2,000 points (or whatever they call the currency) to buy it but you’ll get way more than enough by just getting the 240 stamps you need in Arenas. The achievement will pop upon a Game Over or finishing a stage, whichever comes first.
As for the PSA, due to the systems in place in regards to the modifiers you can buy, this achievement can only be unlocked if you fulfil one of the above criteria with only Modifiers, so no Cheats. (This has been changed in an update; Cheats are ok for this now, though I would still recommend performing the instructions that follow to pop this as quickly as you can.) To guarantee this, enable the Randomizer and head down to the setting Repartition. Press
twice (analogue sticks can’t be used left or right in (at least) this menu for some reason) to get Modifiers Only. Now just select any Hideout and you’ll get the achievement after getting a Game Over or clearing a stage. Make sure to set the minimum amount of Modifiers as high as you can so you have a more likely chance to get Firework, as it appears to be the fastest route to dying by far. Just make sure to move your character a bit so the effect can reactivate after death.