To your unit imbalance thing, Fliers have never benefited from terrain bonuses in Fire Emblem. Healers would be a lot better if their healing power scaled with their level because it's kinda pointless to be healing for 10 when you're getting chunked for 30+.
Edit: The way you're "supposed" to use Lyn is get her knocked below half HP to trigger Vantage and then hope she can one-round whatever else attacks her.
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