If you are positive on strength of the wall won't be affected by the opening (of the final size), I think it can be done by an experienced mason with these steps:
1. Make a template, and cutting a groove on both sides of the wall, using hand held saw.
2. Chipping out the bricks.
3. It will be much better if you can add ring bricks similar to miningman's photo to create arch effect in the finished product.