(Most)Cantors don't attack on EP so you can easily just gang on them early game, after Act 2, it takes a time to see them again(Desaix's Fortress and Grieth's Citadel) and by this point, Bonewalkers and Revenants are barely a threat. You also have Archers who can easily snipe the Cantors, because unlike what people keep saying, they rarely use Supply Tiles, Mikhail in the Temple of Mila will actually move off it and attack someone if they're in range, and in Celica's route, you should have at least two DFs who can easily tank magic and hit Cantors even on supply tiles.
Also, Expel, 65% chance to outright kill a Terror and it's AOE. Maybe it'll fail or just not kill the numbers you need but you have MT to abuse RNG. You can also counter with your summons, which Genny gets really early, she has all the tools to defeat them.
Act 4 shows how non-threatening Summons are because Necrodragons have ****ty Atk and Spd and Gargoyles only have Spd.
I feel like they're a much better way to handle reinforcements because you can eliminate all by killing the Cantor, you can tell right away where they'll come from and only show up at most 8 every other turn.
Which is much better than Birthright's endgame chapters(the one with Iago is just ridiculous) or Awakening's ******* reinforcements