There is no RNG on transitions other than who's going to get targeted with Gaol, Landslide or Plumes. Phase transitions are base on his HP percentages, but a lot of times even if he dips below the HP% threshold he will still use and finish his move if you don't push him towards the next phase after a certain amount of time has passed from his last move. I hope that makes sense.
For example, if you don't get him below 55% within 5 seconds(this number is an estimate, I just know how long it takes) after the bombs explodes, he will use Gaol before he jumps. For the first jump, you either have to delay your skill a fraction of a second while he is performing the landslide if you know he dips below the HP% for him to jump up for the next phase. Or you can just run to the center right away so you can hit him with your next skill without waiting for him to run back to the tank. If you time it perfectly, you can even stand on the very edge and Shoulder Tackle to him before performaing the next GCD skill.
