As a healer how do you help under geared tanks?

[Foo01]

And the black mages, summoners, and dragoons. About every class can run and attack, not just archers. And believe it or not, scathe definitely hits harder than heavy shot (i.e archer's strongest move that's not a DoT and also has a passive that increases it's damage).

When I cast ruin, if I move it is interrupted. If try to cure with my WHM and I move it is interrupted. Only the archer has so many skills/spells that I can use while running at full speed and never get interrupted. I am not sure you can scathe on the move either. But perhaps I'll try some experiments, but I certainly can't cast fire or ice while on the move.