After a 45-minute, closed-door briefing by Los Angeles police on the so-called Southside Serial Slayer, Mayor Tom Bradley said he is satisfied that a police-sheriff's task force is doing "everything within its power" to catch the killer. Bradley said he requested the briefing after City Councilman Robert Farrell publicly questioned whether the Police Department's best detectives had been assigned to the 50-member task force, which is investigating the slayings of 18 women in South-Central neighborhoods believed to have been committed by the same unknown man. "I wanted to be sure that I was not mistaken," Bradley said. "I know the members of this Police Department . . . undertake their jobs with the kind of professional attitudes and approaches (so) that these questions or complaints should not have been leveled." Bradley declined to discuss what he was told in the briefing. He insisted, however, that the task force is not at a dead end and that it continues to aggressively pursue more than 2,500 clues in the case.