Simple enough just lists the ways you would regress somepony
I made up this stuff called foal mix, and be in liquid, airisal form, even in foal powder, the foals would never see it coming then.

Regress... in what way?

If you mean "mentally regress," a strong hit on the right spot and with the right force can do the trick. Hypnosis can also work.

If you mean physically... we saw an Age Spell in the show, right?

3027595 As Daxn pointed out it depends on the type of regression.

Mental regression can be as simple as blunt tramua to the head. There's also hypnosis, and on the lighter end of the spectrum, age play.

Physical regression usually involves a spell or potion. Age spells are canon as of "Magic Duel" though they are pretty high level.