first bolts before the cat?
step one is the exhaust band clamp at the tail of the DPF. soak with PB blaster all around and loosen that up. I believe it's a 15mm. Sometimes you have to use a breaker bar to get it started. Then take a BFH and hammer on the muffler hanger on the back of the muffler and you can slide even the toughest pipe off the back of the DPF.
don't touch the downpipe bolts or hanger bolts or you'll never get that pipe slid off if it's stuck real good!
THEN, once you have that slid off and pushed up, unbolt the two downpipe to cat bolts. They're 13mm head, and you'll need a wobble and an extension for the top one. The nuts are welded to the flange, so no need to put anything on them.
THEN, unbolt the three hangers from the frame rail. they are 13mm head nuts.
Obviously unplug all the sensors, and the cat/dpf will slide out as one assembly.
This is the absolute fastest/easiest way to get that stuff out from under there every time.