This depends on having strong enough units to get to Cervantes but if you don't you probably need to grind. For me I had Robin, Olivia to dance, and Rescues to get Olivia to Robin. There are many reasons given by the map to not turtle. The starting enemy units are weaker than the reinforcements, most don't move unless you get in their range, there are many enemy ambush spawn who cannot be effectively 1-2 ranged since there are longbows, and there's enough reinforcements to overwhelm your units if you're not low-manning. If you push forward there's also chances to feed the weaker enemies to your weaker units, unlike the reinforcements who move and are more threatening.
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