Focus was not in the API in vanilla.
what would could do back then was create a variable called focus
and assign the variable a unit name
/script focus=UnitName(“target”)
/script AssistByName(focus); CastSpellByName(“Fireball”); TargetLastTarget()
Unitname and some others became protected in the API, i think since BC?
You can set the variable still, but you can can not do AssistByName or TargetByName anymore.
There is a focus addon you can get though