After reaching Artorius Throne, go back to Pendrago. Head to Knight’s Tower. You should see Sergei standing over there. Talk to him and you will get the mystic arte.
After defeating Eizen, check the grave nearby and then go back to the inn. Sleep there and then return to the peak to check the grave to obtain the mystic arte.
Defeat Legendary Wyvern.
Obtain 30 Anomalous Orbs, you can get these orbs from defeating mutant hellion (see the side boss/mutant hellion section on the main page). You will trigger a skit at the save point after obtaining those orbs. Go to Lakehaven Height (use save point travel to the entrance of Ladylake). SAVE your game before talk with the merchant (“!”). Now prepare for one of the hardest battle on the game. Open THIS PAGE for the strategy.
Beat the game first! Load your clear data, talk to Rose and choose the talk about taking a break. Go to Ladylake and sleep at the inn. It will trigger an event, now go to the sanctuary. See another event and you will get Lailah‘s 2nd mystic artes.
After getting Lailah’s 2nd Mystic arte, sleep at the inn (I slept at Ladylake ; not sure whether it must be Ladylake or not). Mikleo will suggest you to go to Elysia, so go there and you will see “!” at Elysia Village Gate.