You get by with a little help from your friends....
Here's what some NPCs would suggest:
Approximately 48 hours after your palace experience, you travel on foot (or horse or griffon) to somewhere just beyond the edge of the no man's land. You choose a point of attack, and an agreed upon rendezvous on the other side of the wall.
Then, rather then depending on being able to see eachother, you depend on coordination. A mage of some sort casts invisibility on you, and you begin moving to the agreed upon spot, each with ropes and grapples. You will be unable to see eachother, so timing is of the essence. After crossing the no man's land (as quietly as possible) you all grapple the wall at pre-chosen places, and begin a fast ascent. The grapples (and ropes) may be noticed, so you'll need to ascend with the utmost speed.
[alternatively, you may wish to stagger your ascents so that some characters can provide a distraction while others climb]
Once at the top of the wall, you'll dispatch any opponents you may encounter as quickly as possible, and then move to one of the guard stations/towers as fast as possible. If you've encountered resistence, you'll be visible, so speed and silence are extraordinarily important. Moving to one of the guard posts should aid you, as just trying to cross the inner no man's land will attract undue attention. You'll descend where the draconids do, and move across the inner no man's land imitating them as well as is possible. Xath should be able to help a little, here.
Once into the city proper, initial scouting must focus on locating a temporary headquarters. HQ should be small, out of the way, defensible, and easily hardened against casual patrols and inspection. You'll need to hide right under their noses. Remember, undead, saurian creatures like Yoshi, and Wyverns all roam the area at will, so combat may be unavoidable. Should this be the case, speed and efficiency will be the watchwords of the day--be ruthless, and efficient, and above all, have your results, but not your persons be noticed. Silent weapons are a must--no steeldrakes except as a last resort, here. Justice will have to show restraint with her turning--it is a powerful tool, but easily observed...as long as ity can be done in secret, use it with impugnity.
One of the objectives of this mission is to strike a little bit of fear into our enemies, so any draconids must be made an example of--if the bodies are to be found, they must be terrible, indeed, to strike fear into the hearts of these black-hearted creatures.
After establishing an HQ for further operations, several scouting attempts must be made, all while maintaining a low profile. Primarily, some attempt to map the extent of the sewers must be made--they could serve as a useful tool, indeed. If they are even half as well designed as the city streets, you'll have a veritable network of tunnels from which you may operate, unless the draconids have already taken advantage of them. If they have, you'll find out about it during this part of the mission. Once the undercity has been scouted, a secondary operation must be initiated. For our purposes, we'll call it Operation: Pet Rock (this is the planar ally plan). Communication and supply lines must be established between our forest stronghold, and whatever control point we possess in the city. Operation Pet Rock allows us to do so quickly, quiety, and without notice.
After casing the sewers (or lack thereof), and connecting Oceanus HQ with Forest Base, the next effort must focus on observing movements and major choke points within the city. Where are the draconids sleeping (if indeed they sleep)? How many are gathered in what locations? Do they move in specific patterns? Are they preparing to move? If so, where? During this mission, any time targets of opportunity present themselves, they should be taken, and made an example of. Choose a calling card, and begin to leave it everywhere you have infiltrated.
After having identified major choke points, a secondary (or is it tertiary by now?) plan must be made to hit, and severely damage at least one such point. Your goal must be to hit a target of importance, and wreak such havoc that it will cripple this arm of the invasion, at least until further reinforcements can arrive (or be created). Little planning can be made on this option until our initial intelligence needs have been met. If the target identified requires a larger strike force, acquire and brief said force. Speed will be of the essence, and those people we have trained will be standing by to assist should they be necessary.
Once at least one major target has been hit and disabled (destroyed is preferable), we will have momentary freedom to act while the enemy is off balance. Those of our kin that have already been taken to the palace must be assumed to already be lost to the clutches of our enemies.
The city watch, now serving as a holding cell, must be hit, destroyed, with any free populace within returned to HQ, and through the results of Pet Rock, moved to Forest Base. Once out of the city, our supply conduit (and Oceanus HQ) must be collapsed, and made unuseable for the enemy, unless the resulting escape has been unobserved by the enemy, entirely.
[alternatively, you take a much different track with this last step in the plan. While some characters hit the City Watch (or maybe you have a shadow phoenix force do it), you do something obvious, and get one of the larger units of Draconids to give chase throughout the city. You'll need crowded streets, and you'll need to ditch any subtelty you've developed in the rest of the op to make this one work. Once they give chase (the danger must be seen as small enough to warrant a unit or two, not the whole army giving chase) you take them through a place that is difficult to observe, and cast an illusory switcheroo. Although they should still be chasing you, you'll need to look like Draconids, and they'l need to look like elves. You hit 'em with a few wands of Seeming (or some other mass illusion spell) and take off toward the wall. It now looks like (if you can make it work) a bunch of elves are making a run for it at once--coincidentally, right after a bunch of elves disappear from the city watch. As the good draconids you are, you'll "rouse the alarm" which out to bring your brethren down upon these "elves" with decisive force. Assuming they ever figure out that they're toasting their own people, they'll stop--but not before you've disappeared in the chaos, and left a calling card or two]
With valuable intelligence on the undercity in hand, we ought to have the wherewithal for Pet Rock II, III, and IV (etc.) operations, and have several new allies, as well as new respect from our foe...at best, we can hope that they begin to fear us.
If Pet Rock becomes compromised, it will be collapsed from Forest Base, and escape will have to made on your own terms.
Are there any questions?