Lua's likely the ep boss, meaning an ML in a few months. Probably not Silvana, since Silvana is similar to the Spirit Lords, and the only one of them to incarnate thus far has been Destina. It's possible, but unlikely, that Silvana would come now. We know it's not an ML or SC since we just got one, and the other just drops this week. And this certainly doesn't feel like a summer hero.
Armid is a relatively high probability, but the timing seems a little off. Would have made more sense to drop her before Yulha, maybe even before Taeyou. Since she's dead now and they'd have to do another flashback to cover whatever story for her they need to cover. We've been getting a lot of flashbacks this ep, I think they need to be careful w/ overusing flashbacks to tell the story. Unless the next chapter has her coming back somehow.
I suppose if you go "out there" Armid could be like Meru (living sacrifice/AD vessel) and then you switch up Armid and Lua, meaning this could be Lua, Armid the boss.