The fastest way is to dig up a shadow elemental, sic the dragon on the elemental, and create a UOA macro that casts clumsy, weaken and feeblemind on your dragon (if you are at 5/2 or 6/2 casting you put a delay of 600 after the target, if not, then it will be higher). You have to be in a guild if you are going to train in Trammel, and you will have to command the dragon to kill the elemental once every 90 seconds or so to keep it green. Discording the dragon is recommended.
Every time the dragon reaches a resist skill level divisible by ten, your spells will start to fail. Simply stop casting, and don't command your dragon until it turns blue, or until two minutes have passed, whichever comes first, then you should be able to start casting again. You can have more than one person casting on your dragon, but they will all have to be in your guild if you are training in Trammel, and they will all have to have the same evaluate intelligence skill, if they don't, only the character with the highest evaluate intelligence will be able to cast spells on your pet.
There is a chance that your dragon will turn on the characters casting on it, but all you need to do is command the dragon to kill the elemental and that should take care of it. For that reason it is advisable to train the dragon in an open area, and have the casters stand back a bit. Just don't forget to have the casters cast invisibility on themselves when you are done training.