[Manuk]

You shouldnt cast flame anything with them. I had that problem in a death match. My witch hunter was to cast flame elementals to aid himself but those creatures have not any physical attack but flames so some abysian dude couldnt be harmed, darn.

About getting the defenders in the open i think it will work ok.
getting lots of archers works ok.
flyers work ok.
You can put some fast cavalry ahead in the center so they get through before they reach the gate and they will distract their troops long enough to get some more men behind them and that's all you need.