In the Dynamis LS I'm in my role as rdm is as follows:
Sleepga Mobs- RDM main priority. BLM's are focused on nuking statues and other mobs. Mobs awake they come after the RDM who can take a hit and manage to get sleepga out again.
Refresh MP users- cycle refreshes
Cure: in party im main healer- if whm im party im backup)
Haste- cycle haste to DD's depending on my mp.
I always use Aspir whenever it is up also which is a big help.(cast with some haste gear on)
Sometimes its tough like when im in a blm party and cant miss a refresh on anyone while making sure to get sleepga/sleep2/sleep off on mobs.
Keeping mobs slept and avoiding a wipe is what's important.
I also have convert fully merited so that helps to.
this is my 2 cents on the topic.