Depending on what you're training and what you're training on, the monster skill level may be out of the range of your pet skill level.
i.e. If you have a newly trained greater dragon with 97 tactics, and happen to spawn a shadow elemental with 65 tactics, you will never gain a point of tactics on that elemental.
Contrariwise, if you have a nightmare with 60 tactics and an elemental with 91 tacitcs, you will never gain tactics fighting that elemental.
My guide is about 20 points, plus or minus between your creature's skills and the target's skills.
Either that or just take it out hunting. Do a few champion spawns and your dragon will be perfect.