Support Priests are easy to level :/ *Has level 70 support Priest* The Aco part is the hard part -.- But hey, most other people solo to second class, why is it any different that an Aco does?
Anyway, support Priests have stupidly easy times in parties, they have more SP than Acos. Imagin an Aco attemping to buff and heal a party of 5 or 6 in a difficult area, they'd run out of SP quickly. Priest just stand still and let the SP regain skill kick in, or Mag and continue on. Not to mention they have more skills to help the party (KE, LA, LD...). Anyway, support Priests wouldn't be much without someone to support.
The support Priests I have met while playing on my Hunter were right jerks "I'm not healing YOU" "I have better things to waste my Sp on" "buy a heal clip" "I'm not a heal slave" or crap along those lines -.-