As you might all no the G15 has lots of options to make macro's.
What i have done is Made my G12 button Press. ( Tab then 2 ) this tabs through my enemies and casts my Dots.
Also make sure that the Repeat option is on 'Toggle'.
This can be done for many things. EG
I made one for my G9 that does ( Tab then 2 then 3 ( now waits 3 seconds cast time ) and now 4 ( waits 3 second cast time ).
And with that on repeat it will go through most enemies and cast Dots and spells on them all so all you need to do is get yourself in a good spot with a good healer and off you go!! =D
6):