What kind of magic do you like?

The Nature mage won't have time to cast Antimagic on himself and the Mantises at the same time. In any case, the Mage has quite good abilities. Also, let's not forget that the necessary spells don't always appear in the guild.

Besides, if the Nature mage decides to cast beneficial spells on himself first, like Dragon Strength, the Mage will simply steal them. So, blessings in this case won't be very helpful.

Illusions may create fewer creatures, but illusions cost 3 mana, while Mantises cost 10 (correct me if I'm wrong, but the difference is still significant), which ultimately means that there will be more illusions.

And he doesn't need to cast on the Mantises, only on himself, and only on the Mantises in case of Berserk or Hypnosis, as a removal effect. Blessings are also not needed - they only waste a turn...
Regarding the cost, if you're going to cast something, cast Phantom, it's still twice as effective, and there's no point in illusions at all, you can only make a maximum of 2 Mantises.
Moreover, why should the Nature mage summon Mantises? It's better to summon Elementals (whichever he likes more), in this case, the problem with Hypnosis doesn't exist, and it's enough to summon them once, and then cast Swarm on the Mage, the Mage does nothing, and the Elementals attack, the mana cost is not high.