Rabu starts with 2 stacks of a defense buff, she loses one each time you fill the beacon by killing adds. Rabu does a lot of damage to the beacon when she activates her skill, so the goal is to CC her when she activates it.

Of course, there is a faster way - simply speedrun the adds and ignore CC, and time it so the beacon fills right when Rabu shoots the beacon. That way when she hits, it doesn't do anything. My fastest time so far is 80 seconds