In the end, despite much exertion and grunting, the rock remains in place for the moment. Standing up from his own efforts, the Mongrel representative takes a step back and says. "I think we should be able to do this. We simply need to employ our efforts more intelligently. If we put two people on each side, then two more at the bottom, we should be able to lift it. Sir knight, perhaps if you and the... Uh, temporary Orc take the bottom, we can do this."

Wasting no more time, he and his companion take up a place on one side of the large rock slab, crouching down and waiting for everyone to be in position to begin straining against the rock.

Spoiler
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That last set of rolls was, obviously, not so good, particularly since you cannot Take 20 for this, that taking too long... But you do still have time to make it work. If you follow the plan laid out above, everyone can use Aid Another actions to support one lift attempt, ideally from the strongest person in the group, with a massive bonus to give it a shot at moving the rock. You can Take 10 on both the Aid Another and the main lifting attempt, if you want to. Or, if you have a better plan, you might be able to convince the Mongrels to pursue it instead.