New tank here, only cleared Garuda EX once thanks to the echo buff.

So my biggest problem in this fight is getting the aggro on me while trying to reposition the sisters and spiny during the second phase. I made a thread somewhere and someone replied telling me to use the Enmity combo that the OP mentioned. So the question i got here is how do the tanks know when the sisters will spawn so that you actually have the combo ready when sisters spawn. Is it just experience or there is like a fixed time to it ? Since i am pretty sure the combo will disappear if you don't use it after a while.

Before i know of this enmity combo I usually just pop Fight or Flight and shield lob the sister and pray that healer don't generate too much aggro while i am spamming shield lob. Sometime the aggro got taken by healers, some time it don't.
Also I have problem trying to provoke the second sister off the MT, I usually just use Provoke + Shield Lob to try and take aggro off MT but 3 out of 4 times, the aggro went right back to the MT. Do i need to physically walk over to the sister to do that Provoke and Enmity Combo to get second sister off the MT ? Am i doing something wrong in the things that i listed or is it just because the MT and I have too much gear difference ? I remember MT had 6.8k hp while i only had 6.1k hp pre-echo buff.
Currently I am a PLD ilvl 83 with 6.3k hp. So far hadn't landed in a learning group that let me play MT. So tips concerning Main tanking would be nice too.