Win condition: Rout the enemy
By the looks of things you’ve been ambushed, this time by another Cantor and some Arcanists. The Cantor knows Conjure like the previous one you faced, the difference here being that he won’t use it if you dispatch his forces quickly enough. As such, be sure to do so while still treading carefully.
A mysterious Masked Knight will show up to aid you, and you only have Saber and Celica to work with otherwise, so ideally they haven’t been neglected as lesser-used units. If you can, equip Celica with either a Blessed Ring or a Blessed Sword, giving her HP restoration each turn.
To clear the map, you’ll want to weaken enemy magic-wielders with Celica’s Thunder (which is out of their own attack range) and finish them either with Saber or the Masked Knight’s Ally Phase. Eventually you’ll reach the Cantor – defeat him the same way.
If for some reason Celica and Saber are two of your weaker units, let the Masked Knight do as much work as possible. You’ll want to avoid his death if you can, though.
Finish off the Cantor to clear the map.