/focus [target=focus,noexists]; [target=focus,dead]
/cast [pet:succubus,target=focus,exists,harm] Seduction; Seduction
Once you have the macro setup all you have to do is target the mob and hit the button. After you have seduced it once all you have to do is hit this button and your succ will automatically cancel her seduce and reseduce the same target, you don't have to retarget the mob again since its set to the focus. You can just keep DPSing your target.