Actually, you can generally do pretty well in prison the moment you have soul strike level 10, even as a first job class. You'll just need plenty of GJ (your low SP will run out fast), any hp increase gears if you have them (otherwise, you will die in one hit - it's doable, but much harder), and lots of patience because you will die a lot. However, since it's a ks zone and there's usually a lot of people, you should be able to survive to level up until you're strong enough to level without ksing other people.
You might die in one hit, but you'll also be able to level in one kill, so it's worth the trouble. That's something only the ranged classes (mage with soul strike, acolyte with heal, and possibly bow classes) can do successfully, without godly gears or IC.
