Walkthrough
This mission is a bit tricky, for it needs a lot of patience and logic to solve the way out. You will begin this mission as you are located on the Ancient Training Ground. Go straight to the bridge with Forgotten Ruins located there. When you are there, move forward and jump down to the balcony located at the center of the bridge. Be prepared, though, because down there you will fight those pestering Chimera-infested Scarecrows. After you defeated them, keep moving forward through the fog, which would bring you on Windswept Valley. Keep moving forward again, until the passages will be blocked by a Red Barrier, indicating that you will fight again, but this time, with Chimera-infested assaults. This will be a little tough, as they are able to dive below the ground and dash forwards you unexpectedly. So, if you don't want to waste your Vitality Gauge, style change to Royal Guard and simply perform Royal Block and use a full Royal Release at them (Note: this tactic is useful if you mastered Royal Guard style. If not then simply use Trickster or Gunslinger and equip the Coyote-A). After defeating them simply head forward to the fog. This time you are now on Ancient Plaza. Defeat all of you enemies there, afterwards go through Lapis River (where a bridge is located) and there you will encounter some Mephistos and Faust. After you defeat them, go down to the left side of the bridge (Use the path where you enter Lapis River as guide), which will guide you towards Windswept Valley castle this time. Then enter Ruined Church, then to Lost Woods, defeat the assaults and finally solve the puzzle like you did before. (Opposite the shadow of the post is the right way.) After you finished the puzzle just simply head straight through.
Boss: Echidna the She-Viper
If you already have Gilgamesh and Sword Master Level 4 it'll be as easy as pie but if are just beginning then an effective way was to use first Gunslinger and ready Charge Shot. When she lands down on the ground to disperse her seeds simply charge Divine Dragon (Gilgamesh, Sword Master Level 3) or just use Aerial Rave (Rebellion, Sword Master Level 1). Another effective but weaker strategy is to hit her face with Lucifer (if you already obtain it) and ready your Shotgun. Repeat process when she transforms on her "golden mode". This time it's a bit harder, since her attacks are more deadly now thanks to her seeds thrown directly through Dante. The only way to finish the fight quickly and earn stylish points is to wait until she bury herself deep down, exposing only her body. Shower her with your most powerful attacks, especially Dance Macarbe or Real Impact. Splash and Discipline combined with PF398:Revenge is another sweet method to dish out. Be careful as she can still attack you using her tentacles. After the fight there will be a cutscene with Echidna crying over her goddamn forest and Chimera to be ended by Dante shooting her, obtaining your new Devil Arm Gilgamesh (should you not have it) and destroy the Hell Gate.