You get this as the very start of the game, and thus super easy to get after you beat the game so don't stress.
You start with one purple lumini. You need two to progress. Before reaching the second big blue cube (counting the one you started at), run into some red spikes to kill all but one off. Then reach the next cube.
There's only ONE puzzle that requires two lumini, and it's at the start. Technically you can use two a little further for the spin puzzle and a crystal along the way, but you can double back to the first cube, if you need a second lumini temporarily, or hidden crystal, after reaching second cube, if needed.
Minimal stress.