hahaha i used a crescent wrench to get it off when i did it honestly. i think its like 22mm or maybe even bigger?
Difficulty? undo 2 bolts, wait, do one bolt, fill, do the other. I forget what the stock intake system looks like. I'm not stock so i was able to do it without removing anything. I guess you'll be spending the majority of your time removing the stock parts to get to it. Also i didn't have any of my plastic underpinning stuff on underside the car at the time either.