everyone should be stacking on bottom platform for last phase, and yes if u r on bottom platform for the other phases you will disturb the dps as you are basically blocking them in between you and the other tank so they have no where to move from slam
Incorrect. The tank HAS to tank TFB in the lower platform, especially in hard mode. Not doing so means getting hit by 15k hits, unmitigatable every few seconds. The dps always have a free platform to move to, to avoid the slam: either the one exactly in front of the boss or the one exactly behind it.
And stacking on the last phase is not advisable because all the boss attacks hit the whole platform, so if everybody stacks the whole group is going to get mauled all the time. It may work in story (not sure), but it's quite suicidal in hard mode. What happens is that we usually spread out during the last phase, with maybe just both the healers standing on the same platform.