Gehrman, the First Hunter
After returning from Mergo's Loft, you'll meet Gehrman in the previously locked part of Hunter's Dream. Refuse his offer, and you'll initiate an epic final battle with your mentor.
Understandably, this is extremely difficult and fast-paced battle. While two-handing his scythe, Gehrman will regularly attack with sweeping moves, usually two in a row. You need to avoid both before attempting to attack him, and even then, you'll only get in a hit or two before he leaps away. If he switches to one-handing and his blunderbuss, the combos can last four hits: three slashes and a gun blast. Again, you'll need to avoid them all before attempting to do any damage yourself. Even if he doesn't do the full combo, keep your finger on that dodge button for the duration of the fight. You best bet is to sidestep back from his attacks, then dash forward for a hit or two before he leaps away.
Once his health is half drained, Gehrman will power up, adding even more distance to his leaps. The same strategy applies here, but he'll move even faster. He may also charge up for a massive area-of-effect blast. Retreat if you see this happening. Finally, he'll unleash a shockwave move after leaping and hanging in the air. The best way to avoid the heavy damage is to dash directly under him - the shockwave forms wherever he's facing.