Dementula's boss lair is a circular room filled with eggs that she has laid.
If you come to close to them or you or Dementula break them, they will release a smaller npc version of herself.
Over time one of her mechanics is to break these eggs once per rotation.

Her basic rotation is to:
Spit at you at longer ranges.
Coming closer to her will trigger an unblockable forward swipe that you can easily dodge.
(the red/orange marker that looks like < o > means this)

If you back away after she does this, she will go for a lunge where she is heavily stunned at the landing.
At this point you can do a running leap at her with melee to stun-lock her and get in a few wacks.
Try and get her to repeat the lunge and after a few cycles of stun-locking her, she will break a few of her eggs using a screech for help.
Try and take the egg npcs out while they are grouped and keep a distance from Dementula and within your eyesight if you can, so you can dodge her ranged blobs and her leap.

Around 3/4 hp, she will begin using an unblockable longer ranged forward swipe, where she pulls her long front legs back behind and above her body and plunges them forward at 2-3x the distance she did before in the first unblockable version.
You can also leap stun her when she does this.

At half health, she will begin using a new melee unblockable where she takes her long arms and lifts them high and to her sides like a ballerina about to spin and will do a huge double aoe spin around herself.

You can restore energy after any of her major moves or if you dodge immediately after she spits. You can recover energy in time and dodge the next spit, if you have enough distance on her.