Grenade Ocelot Tutorial for all difficulties
hold triangle, hold dpad up, let go of triangle, hold X, let go of dpad up and x once standing
Get out of the elevator and shoot your gun to the bottom left to hit Meryl.
Getting her attention has her run to the bathroom.
You can quickly grab the box if you want to perform the cave vent gltich to go out of bounds.
iLL_Pazzo & Tromboncino's Claymore phase skip is the new meta to beat Mantis into the phase skip (Meryl falling over)
Explanation for Easy difficulty
https://www.youtube.com/watch?v=8APeemOLizk
Execution on Extreme difficulty
Here's the old method as done by Repeatzzzzz
for reaching the phase skip without the claymore:
For both going up and returning back through the cave
Boba Skip - 50% successrate on first try
Chaff/Stun damage alignment - click picture for fullscreen
More infos about Boba skip can be found here
Further info on the old glitchless Tower A can be found in Glitchless
unequip grenade, three to right for chaff after passing 1-4 turrets
alternative grenade turret tutorial
Before the fight, willingfully fail the lean trick to not require walking back to the start of the arena. By failing the lean trick you will be knocked back further than the regular cutscene knockback.
Visual guides
Ingame view super-wide screenshot (click for fullscreen)