Stuck in story mode, Chapter 3, mission 7-1

Hey everyone.

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Until now I never experienced any difficulties while doing the story. But this one mission hit me hard, and I can't even pass its first fight despit having spent some ressources to up units to be ablt to face it. XP seems to take a while (and I don't know which missions are the best for this) and consume a lot of energy; I don't have a lot of karma left.

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Here are my strongest units :

• 4* Brook, lvl 79
• 4* Grace, lvl 78
• 5* Diana, lvl 77
• 4* Lavender, lvl 75
• 4* Jermiah, lvl 73

Any combination of these five guys gave me a defeat against the first squad. I'm aware I could upgrade some of them, especially the fire ones but as I said I don't have any karma left right now. Other units are 3 stars and none of them are Fire ones.

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What should I do ?