Vlad
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The thing is, in battles against neutral creatures (like Hydras), the SLOW -> EXPLOSION system works very well (but only if you're an expert in Earth Magic). Slowing them down reduces their movement to 3 squares. BUT, if you're fighting a human player, not the AI, this system won't work as well, especially if the ENEMY is also a mage!!!!
When fighting a human player, I always use the Divine Power spell if available, or Righteous Strike when playing as Elves, or something else that improves my troops' stats as much as possible. Then I try to grab something from the other opponent by applying Slow or something else to their strong or dangerous creatures....
But if you're fighting an Undead faction, they can ruin everything with Banshee's Wail...
P.S. This is a separate topic altogether; here, we're only discussing which magic is better. ;)
So, in my opinion, if we're talking about the School of Magic,
Light Magic clearly gives you an advantage. It's very convenient to cast Righteous Strike on all your units.