Template (Focused Practitioner)
Lore and Discipline at Great, Conviction at Good
Powers/Stunts: Channeling [-2] and Ritual [-2] (I'm less sure on the elements/theme for these, but that seems up to player preference)
Now if I've understood this so far, it looks like that means that I get 4 focus item slots. I think (I'm less sure here) that I could use all 4 slots to get a focus item that gave a +4 to offensive control of whatever element I chose for Channeling.
On a more serious note, I'd like to add a clarification to this. As I understand the rules, you have 2
Evocation Focus Item Slots and 2
Thaumaturgy Focus Item Slots, so you can only get +2 to offensive control. The other 2 slots need to be spent on bonuses for Control or Power of your Rituals, or for Enchanted Items.
That said, Enchanted Items are still very strong. An item like Harry's duster that provides a substantial block or armor against incoming attacks 3/session can really shore up a caster's otherwise weak defenses.
In general, though, I agree with most, if not all, of the advice given earlier in this thread. Shape-shifters are definitely one of the top competing builds. In fact, anything with access to higher-than-Inhuman on the physical boosting powers is pretty scary. Even just Inhuman Speed and Inhuman Recovery are very powerful - Speed gets you hit a lot less and makes sure you can get wherever you need to go (whether that's positioning in combat, or just overcoming obstacles like walls, pits, fences, etc.), while Recovery almost guarantees you'll be entering most fights with a completely empty consequence list. Ever notice how often Harry is at a disadvantage in the books because he doesn't get a good chance to rest up between action scenes? Now imagine that wasn't ever an issue for him. Scary, ain't it?
Oh, and do NOT underestimate the potential of Sponsored Magic as an alternative to the Focused Practitioner or Wizard route. Yes, it's still a caster, but the flavor can be very different, and the debt mechanic is quite powerful. Really, anything that gets you Fate Points for free (especially compellable aspects that your GM likes to abuse you with) is a pretty substantial advantage in this game.