Strategery...
I think we need to go through this very carefully to assemble a plan of action, picking out options and goals and then let the chips fall where they may. As a resident expert on all things military (unless someone cares to challenge me, which might be very good in terms of coming up with the best plans etc), here is my take:
Our first goal, being the moral crusaders that we are is naturally to get as many people out as safely as possible. This is our starting point and our end goal.
Second, how? We have two options, so far as I can see, run through the town rallying the people to leave as quickly as possible, staying in the city helping to get as many people out as fast as we can and then leaving with them when it gets too dangerous. The second option is to rally what soldiers we can to fight delaying actions (this is not a distraction), helping those people we can, but mainly trying to buy time for them to escape. Because of the urban territory, this works to our advantage, especially if we can assemble other teams of city guard or soldiers etc to do the same thing. The key is to get small teams, probably around ten to disrupt enemy operations as much as possible. Archers and ranged weaponry are key for this, as they allow for firing from elevated positions or distance, keeping the enemy at distance hopefully until they're either whittled down enough that the melee fighters can finish them off or an escape can be arranged or some other option (traps etc). With numerous teams throughout the city engaged in this, we give the innocents the best shot at escape (where to is another matter). This is the best option in my opinion.
This leads to the next option, do we fight as long as possible and then escape with the rest to leave Oceanus to its fate? Or with the surviving defenders do we go underground and commence a guerrilla war against the undead horde, run either from inside the city (from the sewers etc) or from some nearby sanctuary (probably needs to be mobile to prevent discovery and destruction). If we select the first option, it will probably still require a fighting retreat and I have no idea where we'd be headed. At this point it is the most uncertain choice. The second option allows us to continue inflicting damage on the enemy, distracting them from their ultimate goal (whatever that is), perhaps delay them further giving the civilians more time to escape (though the enemies numbers may preclude that to some extent). If nothing else, we can work on whittling them down a bit, and giving other inhabitants of the island/continent we're on a better chance against those forces we can't tie down.
In the final analysis, we can't win this one right out. This may not be the only invasion force either. We must look at our capabilities, what we can hope to achieve and what our goals are. Given that we have two rangers and we all bring valuable skills in either combat, deception, stealth or all three, I'd say we're all set should we choose the guerrilla option. But we must understand that realistically, we will not be able to save everyone, nor will we be able to defeat this force on our own in a stand up fight, even with whatever forces we can muster from within the city. Indeed, one of the goals of a guerrilla campaign would certainly be to send word with the evacuees to send any available forces to help out with our battle (I'm assuming they are heading somewhere with some sort of military force that can be useful to us).
Okay, I got a little excited, but I think this is a good starting point.
Jeremiah