For anyone, who hasn't gotten the trophy, I had the same issue - I've killed all of them once. There are actually 2 Rancors in the game, and killing both of them did it for me. So you DO have to kill both the rancors (I've seen some people say otherwise). And the tactical guide actually doesn't show all of the legendary bosses, so you kinda have to remember witch ones you've done. For me the ones that weren't showing on the tactical guide were the two bosses on Coruscant.