Spent a whole Saturday morning flat on my back under the kitchen sink last year swapping out a tap, so I know exactly how maddening that brass nut can be. The supply pipes always seem to sit right where you need your hand to go, and the cabinet wall eats up half your swing. What saved me was a basin wrench, the long-handled kind with the spring-loaded jaw at the end. It reaches up over the pipes and grips the nut from above, and the handle pivots so you can turn it even when there's barely any room.
The trick is patience with the first turn. That nut has been sitting there for years and it will feel welded on. Get the jaw seated properly, put steady pressure on, and expect to move it in tiny nudges rather than one smooth swing. Once it cracks loose it usually spins off easy. If the jaw keeps slipping, a shot of penetrating oil and ten minutes of waiting helps more than you'd think.
One other thing I learned the hard way: turn the water off at the mains before you start, not just at the isolating valves under the sink, because those little slots can be worn half round. And have a towel and a shallow tray ready, because there's always leftover water in the pipes no matter how long you let it drain.