A Quality-of-Life PSA for Hard Mode

Tested it to be sure, but after you get Vi-O-La and re-enter Sol Valley, you can get the Upgraded Psychic Beam before you even visit the Flare Pool for the first time.

• All the Green Crystals in the open in Sol Valley (before the 1st rain) are along the edges of the desert itself (only exception is a single patch next to the Tower that you need to Space Jump up to with the Psychic Boots)
• You then just need to visit the only 2 Shrines that have Green Crystals that aren't locked (at the entrance or 1st door) behind something needing an Elemental Shot
  • Ring of Thunder - halfway between the Chrono Tower and the Flare Pool
  • Source of Spring - Directly north of the Flare Pool
• After that, you should have JUST enough Green Crystals to get the upgrade (if you're unsure, check in your Logs

It's made easier by the fact that before the 1st rain, there are no bots roaming Sol Valley, and the Gullbats are still passive, so threats are next to zero.