 Depending on your views about the range of possibility, there is much in this story that you may think is impossible. You may think that it is impossible for there to be Gods who can act as required; for example, depending on your conception of the Gods and their actions, you might think that the story requires instantaneous action at a distance. You may think that it is impossible for force fields to be positioned with unbounded degrees of accuracy. And so on. However, if there is nothing in the set-up that makes you baulk, and if further elaboration of the set-up does not introduce any singularities, then it seems that you should just accept the conclusions with equanimity: Achilles is rendered immobile by conditional intentions on which no one acts, or by a set of force fields none of which he is in direct contact with. Bizarrely counterfactual circumstances have bizarre consequences.