My group was Knight Knight White and Black Mages.
I used this excellent guide:
For me the best way to beat the game within 8h is by not fight at any point in the game except for grinding levels and making good use of temper (black magic lvl 2). Temper will make bosses pretty easy to beat. just keep casting temper on your knights with your black mage and heal with the white.
when you get levistone in Ice Cavern you gonna fight an evil eye and must grind for as long as you can here. it took me less than 1 hour to go from 15 to 35 fighting evil eye.
from here it will be really really fast until you beat the game.
I grinded to level 45 before enter temple of chaos though!
Took me less than 6 hours using this method.