Help with build (Mana Guardian)

Can someone take a look at my Guardian build and see why it does zDPS on boss fights?

it's fine for clearing but A.L.8 map bosses it does practically nothing. Boss health regen > current dps and I dont know why.

I'm not doing cold damage so the EE should be working as intended, right?

What is my mistake, guys?

Frost Bomb + Increased Duration Support can help