My keyboard is full basically and soon are my actionbars too.
I have regular 1-5, QRTFGXCVB and all those with shift too and shift ASD, 4 buttons on the mouse and a few ctrl binds for a few things I use rarely and also F1-F4.
It would be devestating to try and squeeze in 3 scatter binds and 3 silence shots or might my problem be that my keybinds already are kinda bad, I feel very comfortable with them though.
No idea how one does it.
Here is how I do it:
This system is basically light years ahead of the classic way, since I don't need extra binds for Arena123 stuff, I just use the same ability button for target/focus/arena123, selecting the unit by pressing the appropriate modifier (nomod = target, Lshift = focus, Rshift = arena1, Rctrl = arena2, Ralt = arena3).
In my case as you can see in the image I use the Anansi thumb buttons to remap the Arena123 modifiers (rshift, rctrl, ralt), but you can imitate this with any remappable key (with the 1-2-3 buttons of the Naga or G600 for example). Cleary superior way... I'm using 1 button while with the classic mode you need 4 buttons FOR EACH ability, which is insane and completely suboptimal. In Cata there was a 3k rated player that used this system ( Mahiko).
For the ActionBar space issue, with Ion you have infinite ActionButtons and macros (every button is a macro), you can create as many bars as you want with all the buttons you want, it's an excellent addon. If you try it beware that since 5.4.1 it has a bug but it's easy fixable.
Here are my targetting macros, I have removed the abilities I have binded with ctrl and alt because they're not relevant:
/targetenemy /targetenemyplayer /target [nomod] arena1 /focus [mod: lshift] arena1The first 2 target commands make this macro work like tab targetting outside arena (giving priority to enemy players over mobs).
/assist [@focus, help] /target [nomod] arena2 /focus [mod: lshift] arena2The first command (assist) is for PvE, to get the same target as the Tank.
/target [nomod] arena3 /focus [mod: lshift] arena3