Doesn't seem that cryptic to me. As soon as Chris talks about lowering the odds, we see the seven riding into the hills for what's obviously a surprise night attack on Calvera's camp.
Attacking Calvera would raise the odds in my estimation, not lower them, because it would be only the 7 men against the Calvera band instead of waiting in the village and fighting alongside the villagers, bettering their odds.
There's that; but in the kind of night attack Chris was thinking of, the Seven would have the night and the surprise and the high ground in their favor. If they didn't hang around to fight a pitched battle, but just did hit-and-run it might have worked.