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I think trial and error was a poor choice of words, what I meant was even if you dont get the inferences initially in that particular game, try laying out several game boards
Find the premise & conclusion and look for the gap between their support. Whatever AC you pick, if you add it to the stimulus it will make your premises less supportive of the conclusion.
Ultimately weakening is about exploiting an inherent weakness in an argument. There will already be a flaw or a gap, as @c.janson35 discussed. You're looking for that achilles heel and the AC that best exploits it.
Ooh yes, be careful with NA. There is no "trick." Over time you will get better at pre-phrasing the gap. I believe having a firm grip on the gap in addition to the conclusion is the key to these questions.