In PnP the best adversary I came up for the PC group was a halfling. That halfling tore them up and always was able to get away if things went against her way. It was hilarious. Halfling Wizard with Reduce Person, Fly, and Improved Invisibility makes her next to impossible to hit, then she had a nice array of wands of fireballs, lightning, and stinking cloud.
4 out of the 6 PCs died in that fight because they just couldn't hit a lone Halfling Wizard. HAHAHAHA!