Timerra + Ike, has yet to die on a map for me. Great defense and res and speed, and Ike gives her full weapon triangle.
Yunaka + Corrin, avoid tiles at will, draconic hex debuffs plus knife poison really weakens those beefy enemies.
Anna!Eirika, though honestly any mage knight would do well I imagine. Eirika gives you a magic wind sword and being a cav unit boosts your engage art damage by 50%.
Fogato + Sigurd, momentum adds a lot of damage when you add 14 move to the mix