I have a macro on my rogue that actually writes another macro for my fan of knives. I target the tank and press the button that writes the macro. Super easy and I don't care how they spell their name. I'm sure this can be modified to the same effect with the /assist.

1st macro (I've named this one "setTotT":
/run ToT = UnitName("target")
/run if not InCombatLockdown() then EditMacro('TotT', nil, nil, '#showtooltip\n/cast [@'.. ToT ..'] Tricks of the Trade', nil); print('Tricks of the Trade set to : ' .. ToT); else print('CAN NOT DO THAT NOW!'); end;

It makes a macro called "TotT"

I copied this from someone else and am not entirely sure how you'd go about modifying it (also not 100% on the syntax you need for using /assist in a macro), but I'm guessing something like:
"setASS"
/run ToT = UnitName("target")
/run if not InCombatLockdown() then EditMacro('ASS', nil, nil, '/assist [@'.. ToT ..']' , nil); print('Assist set to : ' .. ToT); else print('CAN NOT DO THAT NOW!'); end;

Now you should have your "ASS" macro. =D

Oh, one quick note. Before using this for the first time, make a macro called ASS...doesn't have to have anything in it. It's just to set it up so you can choose the icon and place it on your hotbar. It will get changed to be
/assist [@whoever]