As a healer how do you help under geared tanks?

[Foo01]

It seems that there is a great gear disparity between tanks. It was really apparent when I was running nothing but brayflox normal mode. I see level 32 tanks. Some can take 3 hits and be down only 30% health (i.e. 70% remaining), and others take 3 hits and they are like at 10% health. It makes a world of difference, because I can heal them all I want, but ones that take so much damage means I run out of MP much sooner and then we are all dead. I get them protect, but I am still working on getting stoneskin. But it really does matter which dungeon, so in some case stoneskin is going be denied because of level sync. So is there a solution? Or is it mathematically impossible.