Focus is a measure of how well you maintain concentration when casting a spell or combat art while getting beaten on by a mob. And the only way I know of to raise the ability is to cast while getting beat on!
So, I would find a spot with no aggro and a lot of greens and pick a fight with one of them. Turn OFF your auto attack, (click melee off), and just toss special abilities at them. I think you could even use non-damage abilities like trying to stealth, (which will fail), or attempting to shoot arrows, (you'll get that "too close" message at the end of the cast). Again, the point of the excercise is not to kill the mob but to cast while he beats on you...
Don't HO as that will cause way to much damage, (there is a damage multiplier for completing an HO).
The mob won't do much damage (after all, he's green to you right!) and when he does finally wear you down to orange or even red, you can switch on melee, kill him off, wait for your health to come back up then rinse & repeat!