The Lord of Tzeentch was the first character that i bought, and has since seen the light in a few battles, winning major fights against Dark Elves, Lizardmen and Dwarves. As an Exalted Champion he has led many Tzeentchian armies at 1500 points or less, although he doesnt normally get fielded at 750 or less as his points costs are rather prohibitive!!
As a Champion of Tzeentch he is normally a level 2 Wizard going into combat, with the Lore of Tzeentch. The best spells for him to get are the ones that increase his combat potential, in particular Orange Fire of Transition, which allows him to reroll all his combat rolls! (This includes any Armour saves and Ward saves he may need to make.)
Generally i equip him with either a melee combination or a magic combination. Magic, he gets a Spell familiar, the Golden Eye of Tzeentch and perhaps Enchanted Shield or a Power Stone. Melee he will recieve something like Great Fang (excellent for well armoured heros and troops) or a mix of Gaze of the Gods and Crimson Armour of Dargan. Mundane items he normally recieves are a Halberd (Str 6 WS 6 - 8 hits with rerolls x 4) and a normal shield if he doesnt get Enchanted Shield. This normally provides him with survivability to take on enemy heroes easily as well as smaller units, and enough stopping power in both ranged and melee to make him a threat to the opposing army.
In conclusion, a Champion of Tzeentch on a disc is a fast, tough, and hard hitting opponent. His mobility allows him to avoid unfavourable charges while pumping out offensive spells, but able to attack and destroy vulnerable units and solos like warmachines and enemy spellcasters. Weaknesses are against Combat resolution (he will flee and die) and also enemy ranged attacks (as even he cant make all the saves in the world).
