There are actually lots of guides on YouTube if you're really stuck
I remember chapter 7 being one of the most annoying levels as well but once you understand it then it's actually pretty straightforward. A fair warning though: It's time consuming.
https://www.youtube.com/watch?v=qPinMVTi6Cs
Watch this and try to analyse the techniques.
If you're finding it too hard then maybe consider levelling your squad a bit more.
Personal tips:
Selvaria's crossfire doesn't damage the Edelweiss' main armour so place it in a position to cover your troops.