I moved her to SS tier in PVP tier list. Emilia was previously held back at S+ tier because she needs a decent Ult level to be a damage threat, but I think her utility alone with the Unknown team is enough to offset it.
You can run the Normal stage with friends or AI to complete the mission easily. Note that using any Re:ZERO character gives you a huge damage advantage, so make sure to use if you already have one. Also, heroes with Amplify and Lifesteal effects are highly recommended. Let me know if you've managed to clear this mission :)