Slipping as in it's climbing out of the recess in the pedal?
You could try some plumbers freeze spray on the pedal axle, sometimes that's enough to reduce the force required to overcome the friction between the two parts (by shrinking the axle relative to the crank).
Otherwise what I'd do is to clamp the hex bit into the pedal recess using a vice so there is absolutely no way it can come out, then use something (ring end spanner round the hex bit with extension bar over spanner) to turn the hex bit.
Slipping as in it's climbing out of the recess in the pedal?
You could try some plumbers freeze spray on the pedal axle, sometimes that's enough to reduce the force required to overcome the friction between the two parts (by shrinking the axle relative to the crank).
Otherwise what I'd do is to clamp the hex bit into the pedal recess using a vice so there is absolutely no way it can come out, then use something (ring end spanner round the hex bit with extension bar over spanner) to turn the hex bit.