A half-elf Thief multiclassed Druid (Initiate of the Old Faith feat, grants Wild Shape) with Dilettante + Versatile Master feat for an at-will MBA Beast Form power (Grasping Claws, Savage Rend) can do it easily. Pounce is an option for purely charging, but usually, the other two are better.
If you want to dump Wisdom, you can take the Vampire's Vampire Slam at-will power through Dilettante, and give it the Beast Form keyword with the Wild Dilettante feat. Alternatively, you can take the Druid at-will with the Adept Dilettante feat to change the ability score it uses (Wisdom) to Constitution or Charisma.
Revenants with Half-Elf Soul can do it as well, and their stats match up better for a Thief. However, it takes up another feat slot and you lose Human and Elf feat support. Paragon Multiclassing is an alternative for other races, but it restricts you to the Druid multiclass.
I would suggest taking the Sneaky Staff feat to make use of Sneak Attack and Weapon Finesse bonuses. Clarification, as pointed out: Sneaky Staff wouldn't work in beast form.
Claw Gloves will add more damage.