Here is some advice, after something like 200+ attempts I finally beat it with my 3 jobs (MNK, WHM and PLD), hence I feel I have some ground to offer specific advice, either way please feel free to ask if you have specific questions not covered in this post:
Rules:
Did you see avatar? like in pandora you have to cultivate a strong mental attitude and obey the rules... Titan Rules.
1) It really doesn't matter if you are a tank a DPS or a healer, you are not allowed to get hit by anything except tumults. the tanks, however also have to receive Mountain Buster.
2) Memorize the patterns, not because you can't get out of the way on reaction(I'm sure you can) because you will need them to follow the third rule.
3) Make a macro to announce your party that weight of the land is coming, this is really as far as you can go to baby-sit others in this fight, Rule two is important because you have to flawless, after a few times people will get use to your macro and missing a single weight of the land can cost you the fight, keep that in mind.
If you follow those 3 rules you will stop being a liability and from there is just a matter of time until you find a competent group that can clear it, as others have said playing healer in titan is significantly harder than Tank or DPS.
Critical parts and advanced tips:
The Heart:
There are really just 2 scenarios here, your group either can or cannot break it, the easiest way to tell the difference is: if everyone reached the heart part alive, used the LB and the heart didn't break that group simply does not have the damage or if you failed to meet those condition after 3 attempts your group simply does not have the skill/practice to break it. There are some strategies (Auto-attack during the first 3 phases to get LB lvl3) to break the heart even when you group lacks the required damage. I personally do not recommend them, because damage defines the duration of the last part and you want that to be over as fast as possible.
After Breaking the heart:
This is really the most critical part because Titan will receive you after his special move with the burning rage of a thousand suns, the move order right after the heart is:
Mountain Buster > Tumults (5) > Weight of the land
1) If you don't top your tank right away he will die to Mountain Buster.
2) If you don't top your tank after Mountain Buster he will die to Tumults.
3) And finally if you don't heal through tumults quick enough you will be caught in Weight of the Land.
Long story short, after you break the heart it's time to put on your gamer on face and show that ******* what you are made of.
During this last phase there is only one Gaol (rock jail) in Titan's rotation:
Gaol > Land slide > Mountain Buster > Tumults> Weight of the land > Bombs > Land slide > Mountain Buster > Weight of the land > (rotation restart) Gaol
Gaol: Break it asap.
Land slide: Dodge and get ready
Mountain Buster (Table flip):
@Tank: Precast a defensive ability
@Scholar: Cast Sacred Soil behind titan
@Everyone else: stay behind titan inside Sacred Soil
Tumults:
@White Mage: Medica 2 > Medica
@Scholar: Succor > Succor
@Everyone else: Get ready to run, but stay still until the last tremor
Weight of the land: needless to say but get out of the way.
Bombs:
@Tank: dodge the bombs, if she get hit by them she will die.
Landslide: Dodge
Mountain Buster (Table flip):
@Tank: this is what is going to kill her if she didn't dodge
the bombs.
Weight of the land: needless to say but get out of the way.
Special note for white mages, you have to reach at full MP and with shroud of the saints available to the last part after breaking the heart, otherwise unless you have a bard you will be out of MP before the fight ends. Pro tip, do not overheal the tank, she actually doesn't need that much healing before the last part.
Good luck!