How to easily defeat Dancer of the Boreal Valley.

You must use the spell: Pestilent Mist.
(Spell: Pestilent Mist can be bought from Orbeck of Vinheim)

If your not a caster you can respec by using a bug that lets you respec at Rosaria (who can be found in the Cathedral of the Deep) without using up your 5 allotments or using a Pale Tongue.

You simply respec, accept the changes, then still while in Rosaria's menu, quit the application. When you reload the game, the respec will have saved but your pale tongue / allotment for that NG cycle will not be used up.