If he wants to use the ability a lot, then the Claws [-1] option is a good one. Note that the claws power need not literally be claws, it could be used to represent hands wreathed in flames (in game terms, the damage is the same, but consequences inflicted would reflect burns rather than slashes). Instead of using the Human Guise [-0], however, I'd use Human Form [+0]. The difference is that Guise conceals the power, whereas Form turns it off. Having hands that burn
all of the time might not be a Good Thing(tm). Note that you do
not get the [+1] refresh you'd normally get for Human Form, since it only applies to a single refresh power.
I don't have the book in front of me, but I think there was a 'poisoned' option for claws, which might allow for a lingering 'burning' effect.
As to the other options:
* I don't think the single-attack rote concept works for this. Consider that it will cost a stress for each use; this means you won't get many uses per fight.
* For the biomancy option (suitable for spelled claws but not hand-fire), here is my take:
http://www.jimbutcheronline.com/bb/index.php/topic,20673.msg915259.html#msg915259* For the duration attack I'd proposed earlier, I don't think there are any specific rules that discuss this, so it would be an adaptation/house rule. But I *think* it's reasonable, and its in line with the rules for ongoing magic defenses. And yes, you'd split shifts between strength and duration, so a 5-shift evocation which would normally be a single w:5 attack could instead be cast as a three-exchange w:3 attack.
One last option would be to use a maneuver. This would be (I think -- no book in front of me) a 4-shift evocation that places the aspect "Hands wreathed in magical flame" on you. You could then tag that aspect for attacks (first tag is free!) or even for intimidation.