Thank you 💂!
The trick to beating the mage levels is to utilize hotkeys and control groups and to cheese the AI.
Hotkeys + control groups allow you to select a mage and cast magic missile without too much mousing ( 1 + Q, 2 + Q), which enables you to invest energy in controlling the magic missiles to hit the enemy mages. Also magic missiles do AOE dmg when they hit a target, so you can try to trade 1 magic missile in for 2 of the enemies.
You can cheese the enemy AI a bit here by tricking their missiles to chase yours missiles by getting yours close to theirs so they'll target them and then you can try to slam your magic missile into an enemy mage.
- Edit - This sort of tactics is part of the game being a parody of RTS games.