dark --> wind/light --> ice/light --> earth/light --> (possibly....) LIGHT!!!!
see. you need dark staff as a rdm you get little time to sit and rest.. you need all the help you can get.
next, you can either go for light staff if you heal alot (which i dont.. but i do cast 3's and 4's depending on how good the whm/tank are and when need be)
or, you can go for wind staff, with wind staff, o bless wind staff, it gives you a 10% bonus chance to sticking gravity (a must for soloing aquarious, in fact.. you should get HQ whever possible for any staves) and not only gravity SILENCE which at higher levels means dead party, or living with good xp party.
(see light staff section)
next, you should rpbobly try going for ice, because if you ever want to cast any damage.. this should help alot (with ice staff blizzard 2 becomes my most powerful spell... this also helps because 85% of renkei are distortion) plus its addition to ice damage it adds elemental skill.
(see light staff section)
or, you could go after earth staff which not only helps when you are soloing things if you dont mind switching back and forth and losing TP as most of your damage should be by spells. but it raises chance of slow to stick... its always good when the monster attacks slower.
please note that you should get any HQ staff when you can... if you cant afford much you could possibly just get dark and wind for awhile.you DO NOT NEED a light staff HQ as far as i can remember... but im not looking at my spell list or my change/equip macro page..
gl ^^