Give them all a bloody good overnight soaking with Plusgas then (if you can) try to get some heat on it - small gas blowtorch or similar to 'shock' it free. I have also used freeze spray but this can be a bit dodgy, works at temps below -40C so be careful. If you've access to a welder, try welding another nut onto the rounded one - might pay to disconnect the ECU for that one though...
Try a 12 point socket, they seem to find grip where others fail. Maybe get a smaller one and hammer it over the nut.
Consider or similar, available at most decent tool shops, or screwfix etc. They are a kinda like sockets with curving blades inside made from Tungsten Carbide or something. They are left hand engagement so as you turn the socket it bites into the nut/bolt head harder and harder so will always overcome any rounded head.
If all else fails, nut-spitters or hacksaw. Don't forget the Copperslip when refitting