I keep my Merc upto date with me levelling up, im at the moment lvl 81 and hes lvl 79, altho its quite hard to get the cash when he keeps dying (whoops) when we get ambushed. Its the potions i tell thee... gonna have to get some Rejuvs instead of standard potions.
I used to have it were i would be around level 80, and hed only be lvl 50, in the past, i found that it was far more tedious so i decided to just spend extra few minutes to make sure he was only 1 or 2 levels below me, than 20+