I'm pretty sure @ can't be used with banners; it only accepts unitIDs (target, focus, raid members, etc.).
Originally Posted by CkFromDk
/cast [@mouseover,help][@target,help] Intervene
/tar Skull Banner
/tar Demoralizing Banner
/tar Mocking Banner
Kinda messy, but I think it'll work.
---------- Post added 2013-03-19 at 12:30 PM ----------
If you just want it to hit your focus when you have one (else target), try this:
Originally Posted by QmasterMKII
/cast [@focus,exists,harm] Holy Fire