Manawall was only 2 hits. Everyone makes it sound like its 50 hits or something. TWO freaking hits. The reason he gave isn't a good one.
Let me make this bigger for you: TWO HITS.
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If they had to nerf it, why 20% lol, even manaward is 30%.
I think its more to do with manaward having a trait that boost it to 30 from 20% and the general weakness of the job being physical damage. manawall being 20% follows our armor (high magic res low armor). That's my best guess anyway