I've completed all 8 Elite dungeons, and have Blue QL 10 Talismans in every slot! Obviously, the next step is Nightmare modes! But in order to run those, I have to first defeat the Gatekeeper. I'm primarily set up as a DPS (AR/Elemental if it's important).
How do I defeat this mode? I've gotten him into the 'second phase,' where he floats up into the air after dying a few times to his 'Too Slow' attack, but then he just one shots me after a few seconds. What am I missing here?