PRO TIP: Just keep walking.
Note that the Tomb horror is not a quest requirement. It's not even in the bonus quests. Also, it conveniently can't follow you. You don't even need a stealth ability to get by, you can literally just walk around the corner until you're out of range. Just be sure to set Acina to passive so she doesn't hang behind trying to kill it.
Don't know how relevant this thread is anymore, but thought I'd throw in my two cents since I struggled with this too (until I realized it was optional).
