A guide on how to find and complete the Mezza Lo shrine and solve the "Ancient Trifecta" puzzle.
How to Reach the Shrine
Go east from Ta'loh Naeg Shrine into the forest, then when you reach Lanayru Road - West Gate, go northeast. Keep going northeast toward a large tree on a hill. You will find Kass the Bard next to an orange platform in the ground. Talk to Kass to start a shrine quest and get a hint about revealing the shrine.
You will need to sneak up on a mountain buck (a deer with antlers) and press A to mount it, then press L until the deer stops trying to kick you off. (If you have trouble, try buying the stealth armor set in Kakariko Village, or make some food or elixirs using ingredients that improve stealth.) Then after you calm the deer, ride the deer to the orange platform next to Kass. When you ride onto the platform with the deer, the platform will turn blue, and the shrine will appear.
How to Solve the Shrine
Whenever you hit the crystal switch, a pillar rotates around the large blue light in the middle. Hit the crystal switch until that pillar rotates to the ramp near the crystal switch. Get on the pillar, then use Stasis on the laser. Use Magnesis to move the metal block out of the way of the laser and quickly get back on the pillar that you rotated here. When the Stasis wears off (or when you deactivate it manually) it will hit the crystal and make you rotate on the pillar.
Open the chest to get a Thunderblade. Then use Magnesis on the chest to move it onto the pillar with you. Then stand on the rotating platform and use Stasis on the laser again to make the platform rotate. Use Magnesis on the chest (even if the treasure chest fell off, it should land close enough that you can use Magnesis on it) to put it on the floor switch and open the bars. Then use Stasis on the laser again to rotate to Mezza Lo. Talk to him to get the Spirit Orb.