How do I use multiple superscripts (super, super-super, super-super-super, etc) with them all smaller than each other?

You could use \scalebox command from the graphicx package and make a macro that does this. But, as mentioned by @cfr in the comments to your question, readability very quickly becomes an issue.

\documentclass{article}
\usepackage{graphicx}

\newcommand\aesuper[2][0.90]{^{\scalebox{#1}{$\scriptstyle#2$}}}

\begin{document}

\[
  2\aesuper[1]{2\aesuper{2\aesuper{2\aesuper{2\aesuper{2\aesuper{2}}}}}}
\]

\end{document}

Or \resizebox of the same package graphicx ...

\documentclass{article}
\usepackage{graphicx}
\newdimen\antes
\setlength{\antes}{4em}
\gdef\mb#1{\resizebox{\antes}{!}{#1%
\global\addtolength{\antes}{-.15\antes}}}
\begin{document}
 $\mb{2}^{\mb{2}^{\mb{2}^{\mb{2}^{\mb{2}^{\mb{2}^{%
\mb{2}^{\mb{2}^{\mb{2}^{\mb{2}^{\mb{2}}}}}}}}}}}$
\end{document}