If you're aiming to have everything you'll PK with at 100% the level you get it, you fight easy kill/no match mobs, preferably neutral ones. Razor and above give you way way too much exp. It's also inefficient if you're soloing because of the ratio of time spent in combat vs time resting. You'll probably get more skills gains in an RL hour of fighting things you can tank without issue and resting inbetween, even if the razor mob gives slightly more gains per minute of active combat.

I keep trying to explain these things to people in game but they just don't get it. I saw a dwarf take several RL days to perfect cranial because of this perverse insistence on soloing mobs you can't tank properly.