As an experienced macro user, I know this can be done even shorter:
Code:
#showtooltip
/use [mod,form:1][mod][form:3]Rend;Piercing Howl;Pummel;Thunder Clap
I am always terribly annoyed when I see /cast or /script, they are just long versions of /run and /use. It is a good practice to use Capitalized Characters in spell/item names. In this case, and in the most cases, mod:shift is not needed. Mod is enough. Now you might think, what if he has CTRL+<somebuttonhere> bound to something else? Quote WoWWiki:
"If you bind A to a macro with, say, [modifier:shift] and you have something else bound to SHIFT-A, the SHIFT-A binding will take precedence and your macro will not run." That's about it.