12mm, 6 point socket on my 1/4 inch drive ratchet after breaking the bolts loose with my 12mm line wrench. Once loose, I spun them out by hand, one at a time so I could catch the gasket if it wasn;t stuck to the housing.
Given the location of the bolts/EGR it would be very easy to round the heads using an open end or 12 point, closed end wrench.
I used a small 1/4" ratchet. You loosen the bolts, remove the ratchet and thread them out by hand (there isn't enough space to completely remove them with the ratchet).
The only way you'll get it out is by ignoring any pain receptors in your hands