What kind of magic do you like?

Hero A will not be able to summon demons because antimagic will prevent any spells. Casting antimagic in this case is a losing strategy. Why do people keep talking about using antimagic on a druid or chaos hero in these battles? Because after casting antimagic, they won't last very long; all they'll be able to do is fight in melee. Antimagic is only effective on creatures.

Actually, they can still summon and attack with magic.
1) Summon 4 air elementals.
2) Attack with a fireball.
P.S. All of this happens after casting antimagic.

Antimagic prevents the ability to direct any spells TOWARDS a target, while the target itself can still cast spells.
Antimagic is not beneficial on a hero only because (if I'm not mistaken) it will remove their Potion of Immortality.