To train disco it is good if pet attacks something distant.
If you plan only Disco +AI , train AI last.
Easy (but slow)way is slug pit in underworld. Up to 120.
Another way is Dimetrosaur cross river in Eodon. Little faster , also can train healing. You better have discord yourself so pup will train faster and will be able to get Healing to 120 at the same time.
Fastest way is mini champs with your tamer be archer/thrower. One for tinker legs is nice. Use guard me and attack stuff. Since pet is far from foes each time , it will be pushed to use disco on target first.