If you're the tank party WHM, I recommend having at least 900 max HP. Why? If you go down, the tanks will go down as well. If the tanks go down everyone goes down. 900 HP will help keep you alive. 1,000 HP is even better.
Dynamis is a hostile environment for all jobs, especially if your BLMs are having issues sleeping mobs and tanks are having issues rounding up hate for the large group of mobs that are pulled at a time. Throw a cure or two and you might have a loose mob attack you repeatedly. These mobs hit hard and you can die very quickly. If you can stay alive long enough to cast Cure V on yourself, you'll increase your survivability significantly, even more so now with Cure V's stoneskin.
If you're the tank party WHM, save your mp for curing the tanks. If everything is going smooth in your party and you're sitting near full mp for a stretch, you can help cure the BLMs. If you're not the tank party WHM, you can help cure the tanks and BLMs.
Haste. If you happen to be in a melee DD party, keeping the melees hasted is important. Since Dynamis is a timed event, helping the melees kill faster will help the linkshell get to the next time extension statue and will produce faster drops for the shell as well. If you're in the tank party keeping the tanks hasted lowers their Flash recast timers which helps them keep hate better. However if you run low on mp in a tough situation, the melees can live without haste for a little bit.
Avoid casting anything but cure spells on MNK mobs. In the time it takes for you to cast haste or regen, the tank can die. The AoE petrify from BST scorpion pets has a very long range, so get far away. When the scorpions cast breakga, cast stona on the other WHMs first so they can help unpetrify everyone. If you cast silence on BST pet birds or slimes, make sure they stay silenced. Be ready to recast silence quickly if it wears.
If your ls leaders tell everyone to wipe, don't cure anyone. Don't run. Mobs will come after you, and everyone will have to wait for those mobs to come back and walk past before getting up. If you die a bit early and can get up without being seen by any mobs during a wipe, you can run off and start resting. This takes experienced judgement however and I wouldn't recommend doing it if you don't know what you're doing.
That's all I can think of for now that hasn't been mentioned already. Good luck!
FFXIV Lalafell PHY40: CON29, WVR28, TAN21, THM20, BTN15
FFXI Tarutaru: WHM75, PLD75, BST75, SMN37, SCH37, WAR37, NIN37 (retired)