It's simple :)
He would gladly use griffins and angels, but he can't deploy them because they have nowhere to land within the castle grounds. I was closely monitoring this. If they were vampires or ghosts, it would have been more difficult :)
Added 1 minute ago
Regarding mass spells - his imps have severely depleted his mana reserves; he couldn't even apply evasion to all his units.
I also used a similar tactic, but only when playing the first part - to occupy all the space in the castle so that no one could fly over. I just noticed that there was a space in the corner (which was later occupied by summoned cave demons). The griffins could have tried to fly there if they wanted, but they just stood there as if rooted to the spot.
