[0.983] Fireball and AOE spells

Fireball makes trained units useless for as low as 50 mana minus discount. They all die from one fireball. All the more so trained units are costly. For example, one Defender unit costs 273 production. Five Defender units costs 1365 production. You cast one single fireball for 50 mana and your enemy loses 1365 production. Imho overpowered. Fireball shouldn't defeat trained units in one shot.