(one's) level best
The very best one is able to do at some task or activity. I'm just not good at math, so, believe me, a B- in Algebra means that I've done my level best. I know you're worried about the game today, but just go out there and give it your level best.
someone's level best
one's very best effort. I will do my level best to find your husband. Don't go to a whole lot of trouble. Your level best is good enough.
level best
see under do one's best.
level best
n. one’s very best effort. I will do my level best to find your husband.
(one's) level best
The best one can do in an earnest attempt: I did my level best in math class.