The skill challenge/framework for the battle and the multiple fronts can go a lot of different ways -- and a lot of that is just about the flavor you want to have.
The key to really cool, interesting skill challenges is to embrace the idea that absolutely everything about the basic skill challenge can be changed/adapted/modified to fit your needs.
In your case, I'd look specifically at some key ideas:
1. What does each success represent?
2. What does a single success require?
3. How long does it take to make a single attempt?
4. How can you give the PCs interesting choices to make along the way?
So.... in your shoes, I might try something like this:
- Each success represents incremental progress towards protecting the city by securing a specific location. A failure represents a location that is destroyed/lost/etc.
- Each success requires either a combat encounter, smaller skill challenge, group skill check, or other action to deal with the threat in that specific location
- Each attempted success takes roughly five minutes, which includes some time to move to the location, deal with the situation, and a little aftermath. Taking a short rest during the big challenge will require that the PCs accept a failure in the skill challenge (which may make all skill checks required in the challenge +2 harder, as the tide turns against the party).
- The PCs should be offered multiple locations in danger at nearly every stage of the challenge. The structure should tempt the PCs to do things like split the party from time to time as they try to deal with challenges in different parts of the city
So, to take this a little further......the challenge will take place in broad acts (three of them) and stages (each stage is one "success" in the big challenge that structures the whole schmiel; there are 3 or 4 stages in each act). Each stage is 5 minutes long. There's a finite number of stages; failing in a stage (or taking a short rest) makes the rest of the day more difficult, but there's no limit to the number of failures the PC can pick up along the way.
Here's a sketch of an idea for the first act
Act 1. LF Attack
Stage 1. The first wave of LF attacks the docks. The docks area are full of civilians -- sailors, longshoreman, fishermen, etc. The PCs succeed in this challenge by getting the civilians out of the line of fire. This is a quick skill challenge of a sort -- there are 20 civilians that need to be moved to safety. Every success in the challenge gets one civilian out. A failed check means the PC gets attacked by a LF (roll the attack and damage). Appropriate Skills: Athletics, Intimidate, Streetwise (others possible if the PC explains what they're doing). As an option, a PC can choose to subject himself to an attack to buy a success (make the choice before rolling).
- Success: The PCs get enough civilians to safety
- Fail: A few dozen civilians die, and the docs are overrun.
Stage 2. Additional LF are coming in out of the water, but the local militia is finally organizing to try to fight the LF, but they could really use the PCs to help bolster them -- the militiamen and old and/or young, and universally terrified. At the same time, urchins on rooftops are yelling and pointing out a second band of LF that coming out of the surf to the south of the docks and heading into the grove.
The Choice: PCs can either stay behind and try to help the militia fight, or go try to deal with the LF trying to use the woods to sneak deeper into the city. Or they can split up and try to deal with both -- if the party splits, they can't move back and forth between choices during this stage.
Militia @ the Docks. Skill challenge to support/lead the militia. It will require a total of 8 successes to hold the docks for this stage. Special: A failed check means a significant loss of militiamen (exposed due to a bad tactical decision or fleeing in terror) -- and all DCs in the check get harder (+2). Each round each PC is subject to a LF attack. Failure in this area means the LF are able to penetrate into the interior of the city. Three failures and the militia breaks, each PC here loses a healing surge, and the LF Grunts penetrate.
Flankers in the Woods. Skill challenge to intercept and deal with the flankers. It will require 8 successes to find and pin down the scattering, stealthy LF. Each round each PC is subject to a LF attack. A failed check means a LF skulk escapes into the inner areas of the city. Three failures and the stage ends.
Notes: the PCs can choose one or the other, or split up and try to succeed at both, which in this case may be the better approach, if they can succeed at both.
Special: During this stage, PCs can spend powers to gain advantages:
- Encounter: Gain a +2 to a roll you've just made (remember, short rests during the overall battle will be expensive)
- Daily: Buy a success on a skill roll you've just made
Special: The PCs only earn a success in this stage if they succeed at both encounters.
Stage 3. Blood in the streets.
A third band of LF -- using the distractions created by the fighting at the docks and in the woods to the south -- have entered the city by skirting the manor to the north. The PCs must fall back to the interior of the city to fight a combat encounter. Any Skulks and Grunts that penetrated in Stage 2 join the encounter.
At the end of the encounter, they hear alarm horns blowing to the north, at the city gates -- and the rooftop urchins are screaming and pointing at more LF coming in from the water.
End of the Act: At this point, the PCs probably have earned 3 successes in the skill challenge so far, but they've spent some resources that will be difficult to recover. It's possible, if they split themselves in act two, that they have actually failed in one of the stages
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So, in this act, the PCs make some choices, deal with a variety of challenge types, and face a situation where they end up making choices that will make their combat encounter at the end of this stage potentially tougher. Along the way they're subject to attacks by LF, might spend some of their powers, and even at the end of the combat encounter they have to make the tough choice to take a break and let the docks and walls fend for themselves while they take a short rest and regroup, knowing that resting may help them, but it also helps their enemies.
An outline of acts 2 and 3 might look like this
Act 2
1. Battle at the gate
2. More fighting at the gate, and now they're scaling the walls, too
3. insurgents attack the gate from within
Act 3.
1. Renewed attacks at the docks and the walls -- getting it from both sides
2. Falling back to more defensible positions (Get civilians into Manor houses, temples, and inns, where they can be defended better)
3. Skirmishing in the streets (PCs move through the streets fighting targets of opportunity, end of stage they learn that the Cult leaders are in the city)
4. Chop off the head (PCs take on the leaders of the cult to put an end to the attack)
Keep using the last stage of each act as the only true combat encounter, but use the other stages to build up the story, bleed away some resources, and make those final encounters more difficult if the PCs don't succeed.
If the PCs end up needing a boost -- stages are grinding them down too much and they need a little help -- consider some possible little boons -- maybe in act 3 stage 2 they choose which location they want to help get the people into, and each one offers a different sort of reward:
Temple: Recover 2 healing surges
Manor House: Recover 1 daily
Inns & Taverns: Recover all spent encounter powers
(or something like that)
Anyway, that's just spitballing an idea.
