Combining a slider and superscript

For anyone coming across this now 4 years later and beyond, we finally have access to style in text expressions:

Edit text styles and text properties using expression controls.

 easiest way - two layers

You don't. You simply split up the layers and animate them separately. Superscript is a text decoration to begin with and most fonts only offer hard-coded glyphs for power of 2 and 3, anyway, so beginning with 4 any automatic formatting would fail in most cases.

Mylenium