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Where does the 40% come from? Let's take a level 40 mage and, for example, three fully upgraded schools, with a) Order being mandatory, agreed? b) Chaos (as a very convenient combination), and let's say Life... or Death? Maybe Death is even better with the spell "Hand of Death." In this position, the mage has 200 mana. After all the manipulations, approximately 150 Behemoths remain, and 150 mana, or even less (I checked). Based on this, to create a capable illusion army of 80-100 Behemoths, we need as much as (80/4)*5 = 100 mana. This doesn't even take into account that every three turns, we'll need to teleport the enemy stack for 6 mana to the end of the battlefield or create a new one. But that's not all; the resulting illusions are weaker than the original troops, at least because our mage isn't a tactician, and 80/100 is less than 150. We won't have enough mana, even with the infamous "Magic Burst" spell, to create the necessary number. But, to avoid being unsubstantiated, let's just do an experiment: we'll take a level 40 mage with three upgraded schools, place 1000 Behemoths, and check how long we can last.
I'll remind you that we're talking about a "pure experiment" with no artifacts, just a "bare" level 40 Archmage with all available spells on one side of the ring and 1000 Behemoths on the other.