Secondary skills in Heroes 3.

Defense reduces damage by 2.5% for each advantage in defense.
That is, the reduction will be defense minus the attack of the attacker * 2.5% (if the defense is greater than the attack).
If it is less, then by 5%.
And it does not affect the damage from magic.

The damage from the explosion is equal to SP*75+300 on expert.

That is, in the given example, Damage = 50*75+300=4050.
The health of the attacker is 10.
The number of attackers killed is equal to 4050/10=400.

You seem to have confused something, IMHO.

He clearly confused something. Defense does not play a role when attacking with magic. Defense does not protect against magical damage.

Added 2 minutes ago

Without wisdom, you can't go anywhere, just like without earth and air, even the most terrible barbarians, otherwise:smile33:

You can still survive without wisdom. Because the best part at the early levels is slowing, speeding up, cursing, and weakening. However, in this case, without an expert in one of these magics, they are not very useful.

Added 30 seconds ago
I don't like playing with multiple heroes. On small maps, what's the point if it's much better to develop one and run around the map (the map is small). Well, on large maps, there is no dispute, there you won't have time to repel everyone with one, especially if you are playing against the computer, then you often have to run after him all over the map, which, by the way, I really don't like when playing with the computer.
Another thing is that secondary heroes are not as strong. It's good if you take several equal heroes, but what do you think - is it easier to destroy an army piece by piece or all at once? For comparison (although it may not be logical to compare with this, but the situation is the same) - is it better to take down a crowd of thugs: one by one or all 20 at once? The same is true there. Moreover, as I said, secondary heroes are weaker than the main one.

Added 2 minutes ago
With relatively small armies, it is easier for a mage. However, with large armies, magic is no longer so effective - mages become not very good.