There are a number of ways to approach this (as always)
eg Complete Defeat - buried by falling rocks
Major Defeat - Injured -50% to future rolls
Minor Defeat - Hurt -1 to future rolls
Marginal Defeat - slip, -1 to next roll (only)
Tie - Narator decides
Marginal success - player advances to next "station" Minor success - player advances to next station, +1 on next roll Major success - Player skips next roll (and is "ahead" of everyone else)
Complete success - Player skips next 2 rolls (and is even further ahead!)
2) the "Pendragon hunt"approach
Draw a number of parallel lines on a piece of paper and put a marker for each PC on the middle line. Roll a series of simple contests if the player wins they move forwards,and if they lose they move backwards.If they get off the grid "forwards" they escape, if they move off backwards then they are still in the cave when it collapses. The Krashtkid attacks are treated as normal, but the Krashtkids can additionally trade AP's for moving you back down the table instead of wounds
3) The "One big Contest" approach. - Give the collapsing tunnel a huge No of AP's and then a number of different skills "Ominous rumble 12", "Loud Cracking noise 17", "Earth Tremour 1W", "Falling rocks 5W", Sudden Crevace 7W" "Krashtkid attack 12W", "Falling boulders 17W"or whatever. Players compete in an extended contest as normal (using movement, combat, or whatever other appropriate skills they can come up with) to escape, and can use the normal tools (augments, AP lending etc) to aid themselves or each other. Once they bring the tunnel down to 0 AP they are out safely, any who are reduced to 0AP are "out of the contest" (which may be trapped in the tunnel, captured by Krashtkids, or just collapsed and need carrying out by their friends). I'd pick a reasonably high AP total and not worry too much about justifying it in terms of the skills used. I'd also be tempted,contra the rules, to not reveal the total AP's the tunnel has left on the grounds that they do not necessarily know how close they are to escaping (at least until they can see daylight!
This approach has the advantage that it is easier to fudge - You can set a big threat on the character who has loads of AP's and go easier on the low AP character while still leaving both of them feeling threatened. You can also "fix" the AP total if lucky rolls mean the players will stroll out in a couple of rounds...
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