I think that a good fire mage can kill ALL classes(including hunter/warlock/priest, which i can beat, but with some difficulties)
Generally i start a duel by just polymorphing the guy. Just 2 make the audince laugh at him.I already have mana/health pots +the mana jewel mages can make and also have forst armor+arcane intellect+combustion(i prefer atleast combustion(5)). Then i use pyroblast. Then firebalst. then if they are melee, i use forst nova, then blink, then fireball, and if they arent in their knees yet,, fireblast again. If they are magic, after pyroblast i use counterspell(now they're silenced for 10 secs), then fireblast,fireball, and if they're not dead yet blast wave/dragon breath's(or any other good dps instant spell). This is like a general pattern, but it also depends on the other guy as well. For hunters i usually try 2 sheep their pet then use all my instant spells on him, as they have ALOT of instant ranged attack so basically who is quicker here wins. When fighting a priest/lock/warrior u NEED 2 have insignia of the horde/alliance bcz its almost sure they will fear u . I havent spent too much time on duels but they're a pretty good way 2 waste ur time
PS: check this link if u wanna see my gear n stuff
http://wow.allakhazam.com/profile.html?1707385 