Yuri_G
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It's worse with Shadia. But her main goal is simply to escape from Grol so he doesn't kill her. And defeating him with Agrail's Magic Defense and a Phoenix is a piece of cake...
This is true if you have Magic Defense and a Phoenix, but with a bad upgrade for Raelag (in this mission, he's no longer Agrail), you might not have any of that...
Try not to prolong the time, gather all available troops (as mentioned above, most of them will join you), and use Raelag to break through Grol.
If you don't like the hero's upgrade, replay the previous missions.