Very little RNG required compared to minigame or enemy farming. You need at least 50 or so gems to get started duping, since this trick doesn't work unless you can buy the item.

Reposted from my description:

'To do this glitch, use a storm puff so it'll hit and kill Shantae right where she finishes walking to talk to the shop. This will allow Shantae to move around when she's supposed to be locked in place (This can be done in other cutscenes too, most notably the Dance Genie cutscenes).

The real kicker here is what you can do after that. If you purchase an item while Shantae is leaving the screen, the game will ADD money instead of taking it away! This is trickier then it looks, however, as if you do it too late you won't get anything. Every item has a different timeframe to do it, but generally speaking the lower the cost the item is, the closer to fading out the screen has to be for it to work.

EVEN BETTER, if you mess up this trick and don't get the extra gems, YOU STILL GET THE ITEM.

So yeah, have fun.'

Edit: Actually, all you need to do is take damage for it to work, you don't have to die for it activate, unlike other cutscenes. So it should be pretty easy to do RTA.