This is a very useful macro for any class.
You won't have to switch target if you've got more than one.
Very simple, just hold your mouse over the "other mob" and click the macro.
Macro:
The "other mob" that you held your mouse over will now be affected by Your Spell.Code:/cast [target=mouseover,exist,harm,nodead] Your Spell
Example:
The "other mob" that you held your mouse over will now be affected by Blind.Code:/cast [target=mouseover,exist,harm,nodead] Blind