So, uh. FUCK PIETY.

One word : PARTY

I beat Piety my first try without dying.
My tactic was to get into her fight chamber, lay down a portal, then open up a public party and stay the fuck away from her.

All depends on your build. I've played through the game as a defensive Templar, a summoner and now a shadow and fights that are hard for one character are usually a breeze for at least one of the others.

I remember fighting Merveil on my Templar and thinking they really need to buff her. Then on my summoner who didn't have summon skeleton yet she seemed almost impossible. The summoner has found some stuff to be a complete joke that seems really really hard on the shadow.

I gave up trying to dodge the lightning orbs and just shield-bashed her to interrupt her cast over and over and got lucky. I died once, g/f witch/summoner died 4 times.

Everyone has a different perceptive on easy/hard.