How is it possible...

that those monsters in PvE can hit so hard with there spells? Especially the Aflicted Elementalists bosses. A Lightning Orb did close to 300 damage, while I had 70 armor + WY! on me. After that there came a 250 damage Mind Burn and I almost died.
Other spells that hurt like hell are Spirit Rift (easy to dodge, so they aren't a real threat), Ancestor's Rage and other Channeling Magic spells.

And while dying normally isn't a problem, it is a problem when you try to become a legendary survivor.

Back to the question: How come that those spells can possibly do such high damage?

Because they're bosses.

All bosses in Factions deal double damage with their skills. That's why their spells can do what appears to be impossibly high damage