- Give Troopers a Strength each and no Weaknesses. Give the Corporal and the Sergeant a Weakness each and no Strengths.
- Give everyone a single Development Roll before starting play. This can be used for the usual stuff (weapon upgrade or gaining a new weapon only, since they can’t gain rank as they don’t meet the requirements).
- Give everyone a Weakness to start but anyone can use their Development Roll at the start of play to “upgrade” their Weakness to a Strength rather than for something else.
- Don’t pick a low AA equation from the list (that’s fine for campaign play and part of the curve) but for a one-shot you want something that gives an AA of 7 or 8. I’d be tempted to just go for 8 to be honest.
- Pick a really “meaty” Special Ability for the aliens. There are some in the list that are part of campaign play but aren’t as “punchy” for one-shots. For one-shots I particularly recommend: Ambush, Armour, Exploding Bodies, Lasting Wounds, Regeneration, Suicide, Swarm.
- Roll two “planets” into one session, and prep. two different aliens. Allow the PCs to upgrade between planets as usual (gaining Level and so on). Keep the AA at 8 for the second planet.
- If someone dies before the final encounter allow them to drop back in between encounters with a new PC who has no Flashbacks available (but is fully healthy, with different stats if they wish, etc.).
Be wary that the “curve” of campaign play is weighted towards a lot of successful PC carnage at the beginning (since the party is awash with Flashbacks), then it gets stickier as the rate of Flashbacks entering the group doesn’t match the number being used up, before the advantage drifts back to the PCs further on in the campaign as all their stats approach 10 and Flashbacks come in from PC death. So, fully beefed-up Troopers on mission one against a low AA opponent is (deliberately) like shooting fish in a barrel. Starve them of Flashbacks to start and they will have a rough time.
If I were doing a one-off like Predator or Aliens then it’s probably expected that there will be PC death and a a rough ride.