Play (one's) ace (card) phrase
To use the best resource you have because you think it will bring about a successful result or outcome.
At the end of the match, the coach played his ace by putting the most skillful player on the field.
The American empire thought they could conquer Vietnam by playing their ace card, using Boeing B-52 Stratofortress, the most modern and destructive bomber, against Vietnam.
I played my ace card, using a special searching tool to complete that exercise, which has no answer on the internet.
The verb "play" must be conjugated according to its tense.
This phrase originated from card-playing.