out of one's depth(in a situation that is beyond one's capabilities. "they soon realized they were out of their depth in Division One" (соперники в первом дивизионе им не по зубам) george serebryakov)
armed to the teeth(heavily armed with deadly weapons: The bank robber was armed to the teeth when he was caught. • Both sides were armed to the teeth.)
keep one's counsel(to keep one's own business private; to be discreet, careful, or circumspect in what one says concerning one's own thoughts, deeds, or situation; to keep a secret for someone else КГА)
keep one's mouth shut(I don't have to tell you how important it is for you to keep your mouth shut about all this. • If she had kept her mouth shut she would still have her job now.)