One shot in (one's) locker old-fashioned noun phrase
One shot in (one's) locker is the last opportunity for someone to successfully achieve something.
This game is one shot in our locker, so try our best to reverse the situation.
Tonight he will fight as if it's the one shot in his locker.
Nothing but fortunate chance
If you take a flyer (on something), you take a chance, risk, or gamble on it.
Having only one chance left one can do in order to achieve something
A second opportunity
Very little chance of succeeding in doing something
Locker is a drawer, cupboard, or compartment usually seen at gymnasiums, schools that may be closed with a lock. Bullets/ammunition might be stored in it in this case (the word "shot"). Maybe that's why we have this idiomatic expression.
1. Doing something extremely well
2. Behaving in an extreme or remarkable manner
1. Holly was in rare form during her speech, so she got the first prize in the competition last night.
2. Corrine: I'm going to buy Christmas presents for everybody. Do you want to go with me?
Laura: Oh...My kids are in rare form today, so I cannot go with you. Sorry!