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To be honest, I don't know what difficulty level you're playing on. I also have experience with other installments, but part 6 on normal difficulty seemed very easy to me.
Still, you need to reconsider your strategy, especially on the global map.
One more thing: playing as Haven, I won 90% of the battles only with the Light Elemental (although I think it's too early for the second mission).
Before that, I used this method:
1: Griffins, with reinforcements.
2: Restoration.
3: Improved Counter Strike.
4: Helped the griffins with Vestal Acolytes.
That is, the rest of my units hardly fought. In rare cases, I used the Vestal Acolytes for pacification on particularly agile mobs.