You have to destroy the silver pillar on the left with grenades. It consists of three parts. The first two are not a problem, although they may take some time. Above all, you'll need a flying bounty hunter like Jango Fett. Jump with it to the highest rock, jump and try to blow up the last fragment while flying at high altitude. The grenade won't always explode at the right place, but you should make it eventually. Once the pillar is destroyed, Waxer will appear below. Walk towards him to complete the mission.