A former low-level employee at the law firm Dewey & LeBoeuf will be tried separately on criminal charges arising from the collapse of the once-mighty New York firm and not stand trial with three of its former top executives, The New York Times Dealbook reported on Friday. A New York State judge ruled on Friday that Zachary Warren, who was a client-relations manager at the 1,300-lawyer firm and has since become a lawyer, will be tried after the main defendants. Those defendants are accused of masterminding a scheme to use accounting gimmicks to mask the precarious state of Dewey’s finances on the eve of the financial crisis. The decision sets the stage for two back-to-back criminal trials over the events leading up to Dewey’s bankruptcy filing in December 2012, which cost investors in the firm as much $200 million. This year, Cyrus R. Vance Jr., the Manhattan district attorney, stunned the New York legal community by announcing a 106-count indictment against Dewey’s former top executives — Steven H. Davis, Dewey’s former chairman; Stephen DiCarmine, the firm’s onetime executive director; and Joel Sanders, the firm’s former chief financial officer. At the same time, he announced that his office had secured guilty pleas from seven former employees of the firm, many of whom once worked in Dewey’s finance and accounting department.