It's not as bad as it seems, really.
Dark Souls 3 boss: how to beat Dancer of the Boreal Valley
This boss is actually available as soon as you have access to High Wall of Lothric, just by killing Emma, High Priestess of Lothric. Do you know why we didn't do way back at level 20 or so? Because we're not suicidal, that's why!
The Dancer of the Boreal Valley doesn't have to be a difficult fight, as long as you keep your cool. Additionally, she's weak against lightning and bleed, so that's something to think about when choosing your weapons and spells.
The basic strategy here is patience and care. The Dancer's moves are not super fast and they're well telegraphed, so as long as you roll behind her (aim for her right leg) you can generally avoid them. Except her spinning slash, you'll usually be fine as long as the first blow misses. She has pleasantly long recovery windows; give her a couple of hits then back off for another go.
In the second phase, the Dancer becomes faster and more dangerous. You'll have to back off significantly, meaning you need to get in and out again every time you attack. As such, you only want to have a go on a couple of her attacks, such as after she leaps and lands.
Alternately, if you find it difficult to dodge attacks, you can hide behind a pillar. Seriously. Although the Dancer's attacks can hit you through and around the pillars, if you keep her on the other side of a pillar and then stay as far away as you can, you'll be safe. While she recovers from an attack or when she starts a combo, dart around behind her and give her a smack on the bottom. Bless her poor heart, she just can't figure out what to do about it. Use a quick dagger with a bleed effect to speed this up considerably.
Whether you use the pillar or just keep your distance from the generally slow-moving boss, non-melee characters can have a great time here. If you have a ranged attack with lightning damage, great! If you have a lightning spell, even better! Bring a bunch of ammo or Ashen Estus and spam away.
When she goes down, you can light the Dancer of the Boreal Valley bonfire. You should then approach the altar again in order to open the path to the next area.