Hokay. Don´t feel like watching this turn into a shitfest along the lines of what happened last time, so we´re going to clear this up real quick; 

- "Flying together" can in no way be interpreted as "Flying at the same time." - Especially as the spell is describing how the caster turns into a flock of birds (as in, a swarm creature), not that it matters, since the dome is going to stop them anyway, but it´s worth mentioning. If the writers of the book wanted to say "flying at the same time", they would have used that wording, and not something different entirely. 

- Yes, Erde can create a massive stone dome like that, poof, super fast. As a high-tiered single-element focused character, that is actually well-below the worst he could have done. He is not auto-hitting you at all. 

In short; There´s a dome around you, trapping you inside with Erde. What do you do?

Edit: What Vordak said.