What about putting castsequences instead of casts in there:

Code:
//Macro - the multicall macro
/click MultiBarBottomLeftButton1
/click MultiBarBottomLeftButton2
/click MultiBarBottomLeftButton3
/click MultiBarBottomLeftButton4
/click MultiBarBottomLeftButton5

//Macro1 - Cleanse
/castsequence [target=player] Cleanse,,,,

//Macro2 - Cleanse
/castsequence [target=party1],Cleanse,,,

//Macro3 - Cleanse
/castsequence [target=party2],,Cleanse,,

//Macro4 - Cleanse
/castsequence [target=party3],,,Cleanse,

//Macro5 - Cleanse
/castsequence [target=party4],,,,Cleanse
5 clicks and you're going to get your guy.