Serpentrix is a bitch because the third phase port means it's out of range (> 40 yards) away from the tank and melee. It sucks.

The best thing to do is position someone close to that third hole at the start of the fight. A ranged with an interrupt but not close enough to be gibbed. You just need to interrupt rampage once and that should buy your tank enough time to (manually) move to the boss.