I've done 2 characters through the level 60 quest... The first time, I did what I assume you are "supposed" to do - searched the cave for the boss with the key, swam through the pool to get to the locked door, opened it, dove again to get to the "end" boss, killed him, grabbed the phoenix. Walked out the back exit, and left the cave.

The second time, I walked in, did a few fights, looked to my right and noticed the way I'd exited from the boss the last time. I thought to myself, "Self, that actually looks jumpable". So I tried. Yup, could jump up no problem. Went straight to the boss, killed him, grabbed the phoenix.

So the tip, if you don't want to bother fighting everything in the cave and going through the whole deal, is when you enter, the first cavern you come to has a red (blood?) pool in it. On the opposite wall, there's a bunch of rocks, with an opening higher up. Jump up the rocks, and you can go straight to the end boss, skipping all the fights along the way.