Ive tested this, and it worked all 3 times in Multiplayer (I have not tested this in Single Player)
During the Final Battle against Mother Trinity, save your Photon Blast untill after you defeat her, in that little cutscene that shows her dying, near the last second, relese your PB, and for the remainder of the battle with Dark Falz, you will be 100% Invincible! I used a Midgul PB all 3 times I tested it, Im not certain if it depends on which PB, I would bet not though..