When I leveled my PUP to 99, I found the easiest way to get magic skill gains is to get fast kills.

If you have the mage body and harlequin head, the first thing the puppet does is cast Dia on the enemy you deploy it on.

As we all know, Dia is our friend when it comes to leveling Enfeebling!

Basically what you do, is fight something you can kill fast (get a buddy?). Engage, Deploy, kill. Engage, Deploy, Kill.

Each time, your puppet should cast Dia (at least it was doing that with me).

This should mean that your skill should level faster, rather than waiting for your puppet to cast again.