If you're simply looking for a way to quickly set focus targets, you could just use a macro. I used to do the same on my SPriest to make cleaving to a second target easier (had all my major abilities cast @focus if I held alt)
The simplest macro you could make would simply be "/focus" and move the macro somewhere on your bar. Whenever you press it, it will set your focus to your cuttent target.
You could make it more advanced by making it mouseover, so that you simply move your mouse to the target and instead of clicking, hit the macro.
As an example of my warlock macros for cleave / focus utility:
Code:
#showtooltip
/cast [mod:alt, harm, @focus][@mouseover, exists, harm][] Havoc
Code:
#showtooltip
/stopcast
/cleartarget [@mouseover,harm,nodead]
/use [@mouseover,harm,nodead][] Shadowburn
/targetlasttarget [@mouseover,harm,nodead]
I manually set my focus target, because there are very few fights I need to use that utility.
For my setup on my priest:
Code:
#showtooltip Mind Flay
/cast [mod:alt, @focus, harm] Mind Flay
/cast Mind Flay
Code:
#showtooltip Mind Blast
/run sfx=GetCVar("Sound_EnableSFX");
/console Sound_EnableSFX 0
/run UIErrorsFrame:Clear()
/run SetCVar("Sound_EnableSFX",sfx);
/cast [mod:alt, @focus, harm] Mind Blast
/cast Mind Blast
And to set focus i use a simple /focus macro. The Mind Blast macro may look complicated, but its simply to avoid spam when I macro it with an on-use effect. Insert a /use command after "/console Sound_EnableSFX 0". No, simplicity has never been my thing
As for addons, I'm not sure what your best bet would be.