Prepare your forces by getting them in an assault position. yell "We're going for a switch assault." to get the Armor Class to be at the front.
When the Armor Class (Orange boxes) finishes creating a wedge formation at the front, make sure that the mage class is on the front of the main forces (Red Box).
Have the forces be ready for the assault. Advance on your signal by saying "Advance!"
When close to the gate, get the Armor Class to move to the side of the main force by shouting "Switch, now!"
When the main force is on the main gate, get your pyromancers to throw fireballs and any fire attacks at the gate.
Once the gate is cleared, have the standard soldiers and scouts attack the inside while the mages stay for support.
This is usually for very strong fortifications with a lot of archers on the walls. The Armor Class will protect the front, then protect the wizards when they get to the gate.