One problem right now is that SE has killed off a lot of the aoe damage while also making tanks squishier. This makes it a lot harder to heal the huge pulls especially if you're new to it. I'm a 70 WHM (just hit 70 today) and I let people know at the start of each dungeon that I'm not great at speed runs (unless the tank is a pld who knows their job). Some tanks listen, other run off pull 3 packs of mobs and usually die. Most will adjust after that and the run goes smoothly. The best you can do is speak up at the start of the run and then let it go how it goes.