So i have a weird question, is there a way i could write a macro that shield slams but at -20% turns to execute?
Nope, it's not possible with the current language. They'd have to incorporate something that constantly alerts the macro of the target's health.
Just use a separate hotkey, it's not hard unless you click which you shouldn't be doing, lol. Or if you want, use a modifier like alt, shift or ctrl with the same key you have shield slam bound to...for example you can press 3 for shield slam and alt 3 for execute.