What are you using to attack it with? I recommend a full party healer (Hawkmoon or Belith) and a hero with Spirit Link, for Guardians Kailani as the better pick. If you have Brienne, her special combines with Spirit Link to help turn the party into a killing machine. The other two should be strikers.
Lacking Brienne, my usual party for Guardians is Berden, Kailani, Tyrum, Valen and Hawkmoon. My battle items are minor healing, minor mana, arrows, and axes.
My first time through a final beginner level though, I had to turtle up, putting Tuck, a nearby party only healer, between my spirit linker (Gunnar, since it was Pirates) and Hawkmoon to boost both their healing and mana regeneration.