What are the tools I need to take out the nut of hell on the EGR cooler ?

1/4 inch ratchet, and extension, and a deep well 12mm(14), I can't remember which.
It's NOT that hard. With the correct tools.

 

It's 12 mm socket size.

I managed the nut with a 1/2" drive ratchet (with adapter to 3/8", a long 12 mm socket, and maybe a 1.5" extension. The ratchet wrench was about 18" long, but more importantly, had a swivel head, which allowed to me to snake it in through the mess of cables.

For the associated stud removal, a 1/4" ratchet wrrench with an E8 Torx socket. And that was one sweaty click at a time, took about 15 minutes.