Discussion of the Academy faction (Mages and Wizards). Tactics and their specific features.

Well, if the gremlins destroy the tent, Kaspar's ability will be useless...

No, they won't destroy it. On expert difficulty, the tent will have 800 HP, which means you need 134 gremlins. This is only if there are 2 native heroes in the tavern, and they will also be targeted by ranged attacks.

I understand going for the Necropolis center (with resurrection and a tent), but for the Mage, that's tough...

If you quickly level up the tent yourself, it's possible (+ you have a day in reserve), and there's no other option. Kaspar simply does it more effectively due to his specialization and RD.
But even a few days of delay in going for the center is fatal against Kaspar, because with his vision, he spots 3 artifacts and plans his resource gathering perfectly. With a bit of luck, he can even conquer Utopia in a week, and there's no way to out-level him in the center afterwards.
If you both go for the center at the same time, an interesting race begins, but the problem is that Kaspar goes there more consistently, which is why I think he's more likely to win in this series.

That's obvious, but once I went there because I had no choice; there were mages and scubas guarding it, if I remember correctly...

Yes, mages and an older grade of succubi are a problem, but sometimes Kaspar can break through, while archmages and druids are almost a guaranteed loss, although this is true for any hero and any race.