Your slave toon needs to have assist macros for all of their attacks and spells buddy...

This is what I use on my slave for my 2x Shaman team:

#showtooltip Lightning Bolt
/assist Lexí
/cast Lightning Bolt

Only you would be replacing 'Lexí' with your warrior's name, and Lightning Bolt with whatever spell you so desire your slave to cast. Remember, you have to do this with EVERY spell you want your slave to target your main's target with (all Shock spells, Chain Lightning, etc).

Hope this helps.