I've dealt with reaching the three fingers before, and while it's a bit tricky, timing is absolutely crucial here. Make your way down through the well near avenue balcony into the subterranean shunning-grounds, watching for those nasty poison traps as you descend. When you reach mohg in the cathedral, he telegraphs his blood attacks with a distinct red glow - dodge right as the animation starts and position yourself near the pillars for better spacing. After defeating him, strike the altar behind his arena to reveal the hidden path down.

Now comes the really precise part. You'll need to navigate those descending platforms carefully, timing your drops between the coffins. Once you reach the bottom chamber, called the frenzied flame proscription, strip off absolutely everything - armor, talismans, weapons, the works. The door won't budge unless you're completely bare. Just remember, the moment you embrace the flame, melina will know and leave regardless of whether you've talked with her about the giants' forge. Your only chance to undo this is grabbing miquella's needle from millicent's questline and using it in placidusax's arena, but the timing window for that is pretty specific once you've got the needle.

Watch those platform edges especially near the bottom, a single mistimed roll here can cost you everything you've worked for.