Everyone needs to solo a bit more. That is where the best survival and self-sufficiency skills come from.

A goal every 17+ character should have is to be able to solo the Sub-T all the way to the VOD entrance (with a cleared entrance). While it is easier to do on some classes than others--it can be done on all classes. It's a great goal because it emphasizes a number of key tactical skills--self-healing or avoidance of damage, using the right weapons on the right mobs, knowing when to fight and when to avoid a fight, managing mana, using the right spells on the right mobs. An inability to make this run is great training for where to improve builds and tactics--if one can't do it, figure out why, make some changes and go at it again.