Phase transitions are on a timer or every 20% health on FIREBRAND (NOT stormcaller) .
Best solution is more DPS. If you can't get people to do that, the tank on the ground thing works, or just having DPS alert that a bubble will soon drop on the storm caller tank and to get the F out of dodge as soon as they get DD debuff.
It's on stormcaller actually. Stormcaller is the one who does double destruction, so 3 dps attack him should do it.