The Dancer of the Boreal Valley is one of the toughest bosses you’ll come across in Dark Souls III. When the fight first starts, she’ll move around the arena slowly with only one flaming sword in hand. You’ll be tempted to go in for free hits, but I advise you to keep your shield up and wait for her to make her move.
During the first phase of the fight, the Dancer will attempt to hit you with broad slashes of her sword. You can either block these attacks, or simply roll through them. Whenever she drags her sword on the ground, she’ll summon flames around her which can damage you, so stay back when she does this. Every now and then, she’ll thrust her sword into the ground and perform a flaming AoE attack, so you’ll want to stay away here as well. Your best opportunities to attack during the first phase is right after the AoE attack cools down, and at the end of her attack strings. She’s pretty slow in this phase, you just stay behind her and attack after she does.
There is one extremely dangerous attack that you need to watch out for in phase one too. Her right hand, which will be empty during this phase, can grab you if you’re not careful. The Dancer will perform a grab attack if you’re in range, and slam you into the flaming ground. My character had 20 points in Vigor, so my HP wasn’t very high, and I can confirm that you’ll suffer an instant death even at full health if you get grabbed. If you’re in Cinder form, you’ll likely survive with a very small sliver of health. I ended up putting a few more points into Vigor after that, but it’s still a very risky move to tank that grab while getting more hits in. When you see the Dancer wind back her free hand, either get behind her, or get away.