I've got ~35 hours in No Mans Sky. here's a few pointers I've picked up from that that should help here:
Whilst you're learning the fight, try to drop the stack marker puddles on the edge of the arena and pick a direction to run in (clockwise or anti clockwise ideally). This will reduce the risk of people trapping themselves or leaving themselves with no room to dodge the following dive, a tidy arena is an good arena!
Think about your positioning! Several people have stated this but I'll say it again. If you feel that this fight is luck based, you are wrong (outside of the lottery that is finding people able to do it in PF that is!). If you feel that this fight gives no warning to get out of the way, you are also wrong. Think launch day Titan HM. You just need to adjust to the pace of the encounter and react faster. Everything is absolutely avoidable if you pay attention, stay focused and think ahead a little! Always ensure you have room to run sideways, putting yourself in a gap between two puddles is going to get you killed, wedging yourself with the wall on your right and someone on your left is going to get you killed. Standing 5 yalms infront of him is going to get you killed. Your rotation isn't important whilst RathEX is in the air, so concentrate on what's happening around you instead. If you have multiple ranged, try to leave a gap big enough for Rath to dive between you both so you're not costing each other GCDs.
If you've got a caster or ranged: burn the tail down to 15-20% on the first drop but do not kill it, any extra dps should be on Rath's body. On the next drop, try to burn Rath down to 30% or less. As soon as you've got 3-4 seconds left on the stun, LB both the tail and body. The tail will get destroyed and Rath will run a short distance away, as soon as he has stopped running, LB him again if he isn't already dead.
If you've got 2 tanks: let them deal with the stack marker on them, the healer and dps shouldn't go in if they haven't got the marker. This saves a lot of GCDs.