I have used a 6 inch pipe wrench.  Its small enough to get the end in
between the head of the bolt and the flange but big enough to get some
leverage.  Also the harder you reef on it the harder it grabs.  It destroys
the bolt head but then the bolt is toast anyway.

Normally, to remove these bolts, I use a small screw driver to dig the crud
out of the bolt end and thenuse a small hammer to seat the allen or 12 point
driver into the bolt head fully before I try to twist it off.

Andy