If both melee's are on each side, 1 will hit either of them and the caster sitting right corner or left corner of entrance to define Healers/Ranged positions. This will minimize any risk of getting hit.
The tanks can return to the boss from northwards soon it's drilling in the ground, because "Latency".
Edit:
Fire Lines you need to make a compass direction to where each person goes, except for the 1st water placement. Both tanks face the Fire lines to where the 2nd Water placement goes to destroy it. Make it everyone's direction once you define, don't random it.
(Third Pass)
We have our frozen water at west nipple, Water group goes south, 1st water resist to east, 2nd water resist to South west to exchange thunder with the MT. 1st Water resist can jump right back in soon the exchange is complete, Ultra Flash follows 5 seconds later.
OT takes 1st thunder, passes it to MT, and MT passes to 1st or 2nd water resist.