DC 20 isn't even hard considering the effect it's supposed to have.
Level 7 rogue that just got Evasion has a +3 proficiency and could have 20 dex, so succeeds for ZERO damage on a 12, and never takes full.
I think the scroll should've split between Force and Fire damage. Make the Force damage a CON save for half, because you can't avoid it, only tank it, and make the fire the dex save.
Then rogues still get to use Evasion, but it's not "I anime dodge the comet".