Thunder Rune formation is as follows:
Follow first three steps to receive a Power Ring listed above, make sure your 2nd fastest character is equipped with a Rage Rune, and that you are close to the entrance of Gregminster Palace so that you can exit after receiving the Power Ring. Once outside in the open world, fight random battles and perform the following steps:
4. Let go of enemy units until you get your first group of 3x Simurgh battles and end with a Final Flame spell, defend with everyone else.
5. Next battle, 1x Simurgh, finish with Ball of Lightning from 3rd fastest character's Thunder Rune and defend with everyone else.
6. Repeat step 5 for the battle immediately afterwards.
7. Next battle is 3x Simurgh, finish with Final Flame and defend with everyone else.
8. Let go of 1 battle, fight 1x Simurgh in the following battle and defeat with Ball of Lightning.
9. Let go of 2 battles, then defeat 1x Simurgh in the following battle with Ball of Lightning.
10. Next battle, 3x Ninja and 1x Simurgh. Defend with everyone and finish the battle with Tir's Hell Spell.
11. Next battle, 1x Simurgh. Defeat with Ball of Lightning and defend with everyone else to receive a very hard earned Thunder Rune.