Best Drills to Increase star.

[Toxcatl]

All you need is power train. Use 6x GK drill at once (it's up to you how many times you wanna train your players by using this drill and, of course, how many green packs you have) and use any Hard drill for the last training, the more level drill has, the better result you can get.

Not "any hard drill", but just "any drill". And GKT trick you described here doesn't increase the quality gain. All the drills of the same level have the same efficiency.
It can be used to save some time and to boost lower level drills.

More about GKT trick here