Well if you read Sword of truth series, in that book, magic warfare evolves on a daily basis,
1. one side created a race of creature that's completely invisible, the other side develop another race of creature specifically to hunt the invisible creatures.
2. Enchant wasps and flies with deadly diseases and send then to enemy camp disguise them as normal insects.
3. Use magic to make small glass shards that fly into enemy eyes to blind them, or puncture their lungs, because the glass itself is not magical, magic detection spell will not be alert to their presence.