Suggestions on a player 'raid' on Orc army

[LordPathos]

Heyo, I need suggestions on my next session because, once again, the players have thrown me out of the order I wanted the story to progress.

For the narrative background, I have been running a homebrew campaign and I have been hyping up an impending Ork attack on Sandpoint (where campaign is taking place for now). During some downtime, our Rogue decided he wanted to go scouting into the woods/mountains where the Orks were allegedly gathering. After coming back with the info, they learned that the hills are full of Ork patrols, and the further in they go, there are small encampments, watch towers, and then the eventual fort where the leader is. They also know the attack is 'soon'. The players decide they want to preemptively attack, either to disrupt the Orks or cause the Orks to attack early while not completely prepared. Here is where I am a little stuck on what to do. It sounds like they just want to go in and burn some towers down, maybe kill some patrols, etc. I am not sure how really plan for this.
I've been watching a lot of Critical Roll and I like how Matt does these travelling activities where 1 party member gives a skill and how they would like to use it to progress through the area (stealth to keep quiet from monsters, survival to find natural paths, athletics to hack through thick foliage) and they need to pass X number of skill checks before getting to their destination. Matt then has appropriate encounters for failures (natural traps like quicksand, monster ambush, etc).
I really like this system, it lets the players have a little bit of control on what they are doing as they travel.
I would like some advice coming up with a similar activity I could set up for this next session.
For reference: the location is mountainous woods, getting more barren and rocky as they travel deeper into the mountains, the players are level 4, the enemies I have made them aware of are Orc patrols with random goblins, and captured/trained animals like wolves and rats.

Any help will be appreciated and if you need to know any specifics, feel free to ask.
Thanks!

[Castilliano]

Overview:
1. Prep encounters/events for handy use & improvisation
2. Prep general overview of enemy & its forces
3. Scale DCs based on party's journey through territory.

Sounds like you have a solid list of skills and how they'd help. Maybe write down some DCs that vary by terrain & proximity to enemy (and which enemy). Might also want to sketch out some PC options that make use of alternate skills, like Thievery or Craft to help sabotage a bridge, or social skills to get info or aid. The forest likely has more than good guys & bad guys in it and perhaps some sites of interest to explore, reawaken, fight over, or whatnot.

Prep encounters, even if you don't know when or where you'll use them.
Ex. "Average Orc Patrol", "Unprepared Orc Campground", "Goblin Mercenary Scouts", and so forth.
That'll make the most difference, being able to slip in improvised encounters. Include setting & events if possible, like "Orcs crossing stream w/ wagons" or "Orcs herding victims".

In extensive conflicts like this, I like estimating how many resources the enemy has, i.e. 200 Orc Brutes + 10 per week, 10 Wolves w/ 15 Trainers/Riders (to replace lost), 1 irreplaceable & expensive Hill Giant, etc.
Then not only can the PCs make an impact on those numbers, but you get a sense of how far spread the NPCs can be, how many can travel with supply wagons, etc. Make the numbers so that when spread as you like, normal encounters are sized right for the party.
Or not. There's no reason there couldn't be a mix of easy & hard encounters (especially w/ such a chaotic bunch). Encounters that are too hard really help emphasize the scope of what they're facing (assuming they get enough warning to lay low and avoid combat!)

Speaking of chaotic, how many factions are there? Who's leading them? Who will run to whom for help? As in, when PCs stir up trouble, will they be able to witness some internal fighting, panic, or rage? Who's rallying them that the PCs learn they should assassinate?
I'm thinking the party does not want to trigger the army, yet once you have the situation worked out, it'll be quite easy to improvise and give the party glimpses of the whole pre-engagement.

As for the mechanics, it'd depend on the various stages (like how deep they've penetrated and how many alarms they've triggered) and the group's intentions. So you'll likely need to sketch out a map.
Party heads east, you have X amount of patrols there so determine the DC to ambush or avoid them as the party heads to the fort. It may have an alarm that brings the patrols running back (which should take long enough to give the PCs a long lull to recover or flee, but maybe one group's close enough to make it more tense.) And so on.

As the party approaches the heart, the difficulty should increase w/ density of enemies, and close calls might scare the party away, especially once they learn they'd be exiting through enemy troops.
If the party triggers X number of events, maybe that's enough to incite the army to raid. Being Orcs, I doubt they'd have a passive or reasoned response to having their troops killed.

Hope that helps. Cheers.

[Malk_Content]

If you don't have the GMG it seems they've got a very similair system to what you describe.

Victory Points

If your players seem to prefer more granular and player controlled perhaps setup a small Hexploration segment instead

Hexploration

Personally if you are going to be operating in the area for awhile, with the back and forth of this conflict happening in the same region, setting up a Hexploration map makes sense anyway, as sooner or later players are going to what to see a region map to make plans with.

All of what Castilliano says is great stuff otherwise.

[LordPathos]

I'm liking what I'm hearing so far and I'll just shoot out some basic info for, hopefully, a little more clarity.

The first adventure my party went on hinted at the idea of Orks in the region. On the way to the second, they came across a pack of raiders chasing a travelling merchant. They interrogated one and first heard of 'the boss' that was gathering an army. I purposefully kept everything vague with the hopes that if the party left the town for another quest, they would return to find it under attack and would retaliate, which would eventually lead to what I am planning for now. Fianlly, all they got was the info I gave from the Rouge's scouting mission in original post. But to reiterate, they know the boss is big, hungry, and bored of his current 'participants'.

The boss is going to an Ogre Boss but with the behavior of a Glutton. I also had the idea of if and/or when they get captured (maybe because they try to assassinate the boss or fail too many of these progression checks), he throws them into a fighting pit and they fight 1-3 lite fights before they fight him. Depending on how beat up they are (I don't plan on throwing them free healing) I may tone down the Boss a level or 2. But I wanted it to be tough, really push the threat of death their way. They have easily ripped through all my bosses with hardly any effort.