If the enemy has only summoned units left, they should lose
You kill every single unit the enemy brought to the fight and the battle ends in a draw because they still have summoned Starchilds or Primal Remnants running around. That shouldn't happen. Summoned units are a mana investment, not a real army. If they're all you have left, you lost.
This is a massive issue in Arena mode specifically. The whole point of Arena is that both players draft a fixed army and fight on equal footing i.e. attack/defense stats, troop choices, sacrifices all matter. But mage heroes completely bypass that logic. They don't care about casualties or stats because they can just summon replacements. At worst they force a draw and walk away with no real punishment for losing their actual army.
Summoned-only army = defeat, simple.