The FireKing frowns for a moment. "Bah, first I am too rash and then I am too cautious, That wizard must have shook me up more than I thought."
"You are right Druid, we must strike. The only true defense Is to leave your enemy as nothing but a broken corpse. But I am obviously too upset right now to properly plan. I need to think and to rest."
His stomach growls
"And I need to eat

"
The Dragon arrives soon after at his lair, stopping only to grab a couple of peasants from a nearby village for dinner. After eating and resting on his hoard he seems much more refreshed and his usual cunning self.
"I believe we should strike. I don't know everything about this wizard or the cities defenses but in war one must take gambles."
"As for the army" The FireKing grins just thinking about them. "They are nothing, not even a threat. Pikes and phalanxes are useless against flying creatures and a magic missile can't reach as far as an arrow. We may not even need our whole army to destroy these fools."
With that The FireKing outlines his plan. (The rest of my post is ooc to better relay the tactics involved)
You open the mirror into your lands and summon all your men through to my mountain.
We and our combined forces rest and prepare for tomorrow. Focus on offensive spells, buffs for yourself (You need them more than I do), and healing spells (we'll both probably need some by the end), Possibly some summon spells as well. Air elementals would fit into the plan well and we should have plenty of time to prepare. Also make sure you have at least one shapeshifting spell to use on me and a way to dispel it.
On day two, we act. I'll start by using the mirror to watch the city, paying special attention to the sorcerors and the army, as well as the wizard. I'll also show my watchers how to use the mirror and make sure they are familiar with the city.
Late in the day, after the soldiers are tired from training and the sorcerors have used up their spells, probably around dusk, we begin the attack.
First buff yourself up and cast elemental swarm. Then make me smaller to fit through the mirror.
We start by opening the mirror on a mountain by the city. Preferably near the city but out of sight. By near I mean a little over a thousand feet, just out of arrow range. You, me, your pixies, and my mephits will go through this gate. You dispel the shape on me and get on my back.
We take flight and head for the city. We will head for the gatherings of sorcerors. We will rain magic and arrows upon the sorcerors, staying just out of magic missile range. About 120 feet up, maybe a little more to be on the safe side. After we have destroyed the sorcerors we will move on to the army. Lowering our hight to make use of my breath weapon and your closer ranged spells. We will destroy them as well.
While we are doing this the previously mentioned Watchers will open a new gate at ground level in a courtyard or road in the middle of the city. Since it takes ten minutes to refocus the mirror we should have plenty of time to get started before this new gate opens. Our combined horde will rush through this opening to begin fighting. The airial forces will move to support them.
The gate in the city will remain open. It will be defended by half of my kobalds. 50 on one side, 50 on the other. As well as any forces you wish to appoint to that task. If the gate is assaulted all units will move to support it if able.
If the wizard ever leaves his tower all units will move to attack him. I will dive towards him and attempt to start a grapple. If I don't succeed I will move back to use a breath weapon and all the mephits and pixies will pepper him with arrows. 95 arrows a round will wear down any magical protection. If he comes with guards I will use my breath weapon on the guards first.
If the wizard doesn't leave the tower we will leave him for last. Then try and destroy the tower using magic.
If, for some strange reason, we are outmached we will fly up as high as possible and move away. Even if they do have flying units they will be outmached with no support. All ground units will retreat to the gate and go through. The gate will close as soon as a serious threat approaches it, regardless of our men on the other side.
When we win we will loot the town as thoroughly as possible. Then we will desecrate it as thoroughly as possible. We will use the mirror send mangled corpses from the army to nearby cities, showing what happens to those that oppose us. If you can cast any spells that will cause havok on the corpse it will be all the better. We're keeping the wizard's corpse as our first trophy of the war.
Edit: Two final notes. If we are attacked by archers all units will move to attack the archers. I will use my breath weapon as much as possible on them.
I will attempt to shield the druid with my body at all times. I have way higher AC and a bit more hit points. I also don't need to concentrate on spells.