My current party have 2 bishops as the only magical firepower(round lvl 10 at the mo). The trick is to concentrate on 2 out of Wizardry/alchemy/psionics/divinty as far as I have been able to see. This is to get access to spells as quickly as possible. Put exactly 1 point into fire/air/mental/water/earth/divine before putting the rest into Wizardry/alchemy/psionics/divinty. This is because it seems like it is MUCH harder to get the first point.
Then you have to get the spells that you can cast in non combat situations to train your skills. Spells that seem to be particularly effective is knock knock and charm. I was able to quickly go from 5 in earth to 90 in earth using knock knock on a certain vault door
Be aware that the bishop using this schema is pretty usless early on. There also seems to be hopless to train skills before you reach somthing between lvl 6 and 8 then you can train as hell.....