Picking targets for buff spells

Am I missing something?  When I cast a buff spell, (even the embue spell, which you can cast early on,) I seem to have real trouble selecting which unit I want to cast it on.  If I click on a stack of units on the map, the spell seems to always apply to the leader.  The only way i can control which unit to cast the spell on is to separate the unit out into it's own stack, which requires a bit of manipulation and multiple clicks.  

A very easy solution to this problem seems to me to be to allow the spells to be cast on the icons for the units in the stack that show up in the toolbar.  This seems so natural to me, that I keep trying to do it, and am surprised when it doesn't work.

I may be missing some existing interface feature.   If so, if someone could enlighten me, I would appreciate it.