1) Covered by other posted
2) All jobs have equal party invite potential, as EXP has changed pretty drastically. Can go into it further if you want, theres lots of info on how exping is done nowadays. Short Story: You will be done completely with ExPing in days so its not really ever a consideration.
3) In Abyssea, most jobs solo equally well. DRK is not one of those jobs tho, it jas very little solo potential compared to, like, ANY other DD.
4) /nin is unnecessary. /war 90% of the time, /sam is situational (proccing, voidwatch, ext). /dnc is useful for all melees, as well.
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