1) make a macro that is just your assist/target stuff and put in a button
2) put /clickbutton ButtonNameHere
/cast rest of stuff
in other macro
3) ???
4) PROFIT!

thats the gist of it, the details are in the focusless/leaderless wiki about 2/3 of the way through
essentially your spellcast macro calls clickbutton which runs another macro and sets up your assist/target target, and then
spellcast macro casts spell