Preparing a One-Shot is considerably easier than preparing a campaign, but often harder than preparing your average session. In this case, I'm going to cheat a little bit in that I'm going to take materials from my campaign to save myself time on creating the one-shot.
Most one-shots, at least for A Time of War and its predecessors, focus on a combat engagement. These are rather easier to come across than battles in campaigns, since you don't need to give the PC's the choice to engage in combat; you just drop them into a hot zone. In order to create one of these single-engagement one-shots, though, you need three things: PC's, and OpForce, and Terrain.
In A Time of War, PC's take a long time to create -- at least 40 minutes each, even for an experienced player -- in my experience. Fortunately, for a One-Shot PC, you don't need "backstory" or "non-combat skills". Pfft! What you need is Small Arms at a reasonable number, some martial arts, and a gun.
Well, it isn't quite that simple. Generally speaking, the players take care of party composition for you when they're doing character creation. In this circumstance, you as the GM need to handle creating the characters and making sure that the party is balanced -- you'll just be handing out sheets at run-time. As such, we need to look a bit at the mechanical needs of a party, as well as the psychology of your players for a moment.
In the interests of full disclosure here, this is based on my experience, not scientific observation; I have a degree in Computer Science, not Psychology, so take this for what you will.
Most players when they play in an RPG will want to operate as a team, but they want their character to be unique or special in some way. A player wants a circumstance to arise in the game where everybody turns to them and says "Do your thing," and they get to stand up and be awesome. In campaigns, there are often whole sessions dedicated to these moments. In one-shots, you'll have a few minutes at the outside. Even so, when we're building characters, we want to make sure each character has a unique ability, and we want to make sure that ability has a chance to be showcased during the scenario. So what abilities do we have?
In this scenario, there's going to be a lot of gunplay; more than I want to do with only one specialized gun character; everybody should have Small Arms and a way to contribute. That said, it may make sense to have one sniper character, who can target especially important NPC's and eliminate them from the second floor of the house. That will give us one specialty.
A healer is a necessity in any combat game, and A Time of War is no exception; a character with MedTech and perhaps Surgery will not go amiss.
One of the most horrifying things for a group of players to come up against in A Time of War is vehicles; their armor just works differently and far more effectively than the player's, and it makes for a great "Oh Crap" moment when a small vehicle that would be laughed off the field in BattleTech makes an appearance in A Time of War. As such, we'll want at least one character with Support Weapons so that when we throw a vehicle at the party, they'll pay attention.
Martial Arts and Melee Weapons can also be niche skills; a massed charge of grunts across an open plain to get to the house will allow players with machine guns and assault rifles to kill most of them, but if a few get through, you can have your Melee expert dispatch them in close-quartered combat inside the house.
Finally, there's Archery, Demolitions, and Thrown Weapons. The first and the third are potentially, but not certainly, useful to this operation. Demolitions, though, provides two opportunities; a grenadier, and pre-positioned traps for the NPC's. Both of these will be very satisfying the for the player using them, since they have immediate "bang" effects that dramatically impact the battle.
Next, the terrain. For a siege like this, we want something somewhat defensible -- wooden walls should do the job. The internal arrangement of the house can be fairly simple. Three rooms should provide the need for characters to reposition to get lines of fire on new threats, but not become cumbersome in tracking. The existing map for Clark's ranch should work nicely as a terrain set. I can combine both of these together and render them on a MondoMat for the game itself.
Finally, the Opposing Force. Again, your rank-and-file baddies can be torn directly from the pages of A Time of War. The trick will be not over-equipping them -- these guys are supposed to take many losses for each one the PC's take, so they should have plausible but impractical weapons for their attack. Assault rifles, for instance, have their Medium/Long Range break at 75m, about the distance between Clark's Ranch and the nearest practical cover. By giving the NPC's assault rifles as their primary weapon, we can greatly reduce their effectiveness when shooting from cover, while still being a credible attack force. A laser rifle, on the other hand, will give the players pause and possibly occasion some attention from their sniper.
I recommend drawing up a few generic soldiers with varying loadouts and deploying them as you see fit; if the players are having too easy a time, add more enemies, if they're starting to drown, dial back on reinforcements. The flow of these grunts towards the PC's is the flow which the major events of the one-shot will sit on.
We need one pre-planned event for each PC type. I'm going with these:
Sniper Duel (Sniper): There's an enemy sniper who is peppering the party with fire. He has a high stealth check, so the party's sniper has the not-inconsiderable task of finding him. The enemy sniper isn't placed on the board, but only has his position indicated when one of the PC's makes and opposed Perception/Stealth check against him. Every two or three rounds he goes undetected, he takes a carefully aimed shot, with a much better chance of hitting a PC. The PC's should fear this character, but be relying on their own sniper to eliminate him. There is also an opportunity here for a scripted friendly NPC to be shot and require medical assistance, both to drive home the danger the sniper represents, as well as give the Medic a guaranteed chance to practice her skill.
Man Down! (Medic): I don't expect to have to engineer this one: at some point a PC is going to be hit, and the Medic will have to tend to that character. As noted above, an NPC can be provided to guarantee the need for this skill, especially if the mission is to protect that NPC.
The Big Guns (Support Weapons): Near the end of the encounter, the bad guys roll out their ace in the hole: an assault vehicle. This would be a light attack vehicle, likely a scout or similar in a BattleTech role. If we presume this is Terra in the early-to-mid 3070's, something like the Minion Advanced Tactical Vehicle (Technical Readout 3067, p. 8) might be a bit out of place from a strictly canon perspective, but fills the role we need. While the rest of the party takes cover from the pulse lasers, the Support Weapons character takes out the vehicle.
It's A Trap! (Demolitions): This is more player-driven. Allow the Demolitions character to pre-position command-detonated mines on the map before the siege, and let them to trigger them. This can be made into a scripted even by forming a large, potentially overwhelming attack that runs right over the planted charges. Boom. The day is saved, thanks to our friend, the Demoman.
Of course, there's the minutiae of actually writing up the sheets for the OpForce and the PC's, but this has already gotten rather long. Tomorrow, runtime considerations!