With time you will learn that attacks aren't random in the game. Most bosses will begin with an aoe damage then a tank buster (or the other way around) with generally 10 seconds in between at least, then there wont be any for 30/60 seconds, generally there will be a mechanic in between like dodging circles , a stack, a spread, something like that. So in general you know how 90% of the bosses in the game will work even without knowing them.
When you get used to that, you'll see that bosses are on a timer: if he does an attack at 15 seconds into the fight, he'll do that attack everytime at 15 sec into the fight, no randomness outside of 1 exception: bosses with 2 patterns, very rare but it goes like this:
pattern 1: aoe - tank buster - fire mechanic - wind mechanic - aoe - tank buster - water mechanic - earth mechanic - aoe - tank buster
pattern 2: aoe - tank buster - water mechanic - earth mechanic - aoe - tank buster - fire mechanic - wind mechanic - aoe - tank buster
As you can see ... not very complicated. In most cases there will be 10 to 15 seconds at least between 2 sets of damage so you can basically heal like this (each 2.5 is basically your GCD/cast time/time of the fight):
15sec: aoe damage
17.5s: you aoe heal
20sec: you aoe heal - everyone is full!
22.5s: Nothing to do so attack spell!
25 s: same
27.5: same
30 s: boss do a tank buster!
32.5: heal the tank
35 s: put a regen
37.5: damage or whatever as there won't be anything for like 10 seconds
As you can see, everything is scripted and there's time between things, you only need one or two heal to heal someone up.
The other advices I can give is: first, use your ogcd more than gcd if you can, they are free and strong: asylum as a whm just for the tank is normal, the potency is stronger than a cure 2. Second: on the same vein don't be afraid to always cast, if you need to heal just cancel the previous cast by moving a bit, nothing's urgent or it's already too late anyway and prob not your fault. Third: when you dodge thing always move just to be outside of the aoe, do not move 3 meters further than it, it doesn't spread and you can't cast if you move. Fourth: try to always stand behind the boss, you will have most people in range and the better they are, the more they'll stay behind too so aoes and stuff are always stacked and easy to dodge/heal.
Hope it helps, it'll come with practice!
