You need to complete that quest (and a few more storyline quests) to be able to enter Sastasha (1st dungeon) in the first place actually. It can be especially tough for Marauders since they can't get any subbed abilities from other classes until release, but it's a pretty hard fight no matter the region, so don't feel like you're a bad for having trouble. Or maybe I'm just a bad too, which is entirely possible. I recommend grabbing some Hi-Potions if you don't have any, and you should definitely make sure you have Antidotes ready as the demon mob inflicts a nasty poison. Leveling to 16 isn't too hard if it comes to that or you just want any edge you can get before trying again, I recommend bouncing back and forth between FATEs.
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