Arcane Blast vs TW Murozond

[Enimonemo]

I haven't found any info about this on the internet, but as I was doing some TW dungeons, my Arcane Blasts usually hit for 1-2k damage and 4k+ for crits. Now here comes Murozond with the hourglass fight, and my Arcane Blast constantly hit him for a measly 500-600 damage with Arcane Power buff! Does somebody know the reason to this? Is this a bug?