Tried many setup for the Glich, some tried to store an hyper dash, other to glitch the map while jumping into the water. These were a little hazardious and I did this quick setup instead.

What do you need ?
Dream Nail and only Dream Nail.
Hyper dash isn't required anymore but Dash could be use instead.

You may need to place a warp (Y+Down) first but not really sure of this part.

First, you need to glich the map, so instead of jumping into water and get the good timing just do:
"Quick Map", "Inventory", "Quick Map" quickly

This should open the Map but the Knight can move freely behind it.
On a controller this should be "L1", "select"', "L1".

With the map open, then open a portail with "Y+Up" but when the Knight is in white and about to teleport, Close the map (L1 or select).

Do not release "Y+Up " when closing the map !

You should see the animation of the Dream gate canceled by closing the map. Just press Dash (R2) or just move to get out of the portal. (If you release Y+Up before moving, the animation will resume and you will get teleported).

If doing fine, you're now intangible and can't get any hit. The game should be a little lost if you fall but with a little time will get you back on feet. Beware than you can lose your invicibility if you go to a bench or use the dream nail (you supposed to be blocked in its transition).

To summarize:

• Map/Inventory/Map - L1/select/L1 > Kight should move behind the map
  
• Y+Up to Open the Portal > Knight in White about to teleport
  
• Close the Map whilen keeping Y+Up to cancel the animation
  
• Dash or move and get invicible

Enjoy the game or break the final pantheon. It's up to you ! :)