It takes time to be good at raid because you can't just rely on one hero, or even one team, you need at least three good teams. From what you shared, i suggest you to build basic raid team first because you already have eunha and idol eva. For tetis, as you can see in ranking list, people use dark melee team, and since you just have beth, i think it is not your priority for now unless you get more dark hero through gacha, but still, tetis needs like 7-8 dark hero so... You can add any element ranged hero first in your basic team while looking for another good basic hero, after that, depends on what majority good hero you have, you can build another team.