Can you give a realistic indication of just how much time it should take? Because even with a reasonably engineered Python it seems to take me hours of mind-numbingly boring shooting NPCs just to progress by a percent or two through Expert. I did try a FDL but it wasn't any better. I know I'm not doing it anything like optimally (and I refuse to put myself in a position where I might expect to lose a ship), but even so it feels like it would take me weeks of doing nothing else to get through Master.
I can give you a rough estimate about how much time it would take me to go from Dangerous to Deadly (0%->100%) currently, in a Courier: 20-25 hours. However, this is subjective because it depends on my performance, in my chosen ship, so yours might be slower or faster. Plus I don't know if going from Harmless to Dangerous takes less or more time than Dangerous to Deadly. However, it seems very likely that you're doing it wrong: like Jackie said, you pretty much have to take out ships that are at least your rank. Ships below it will give you very little rank progression.
For more exact details,
check the ED wiki article on combat. That should give you a better idea, although the rank requirements aren't exact.
Most importantly, if you find combat in your Python boring, I would ditch it. (Personally, I find combat in a Python boring too.) You have a choice to make: do you want to go full speed at rank progression, or do you want to go 75% but have fun? If it's the former, use an FdL, your Python or one of the Big Three. If you'd rather have more fun, then I would very much recommend a smaller ship: a Courier would be in my opinion the best if you have access to it, or a Vulture if not. The Courier has excellent flight characteristics and is a joy to fly. The Vulture has very good characteristics too, and situationally more firepower (but less choice). Both ships are also more fun to engineer, because they give pretty good rewards for your effort. There's also the added bonus that should you make mistakes and end up staring at the rebuy screen, you'll have to spend far less credits.
Also, I'll assume that you'll want to do combat solo, but I would recommend winging up with at least one friend. Not only is it more fun that way, but also quicker and safer. Grab a friend or two, grab some Vipers for yourselves and go hunting. The AI (too) has considerably more trouble dealing with multiple targets than with one, so you can expect to get away with more - and you have each other to help too. In fact, if you'd like to wing up with me sometime, I'd be available for it too; I'll be in the bubble until 2.3 hits anyway.
From here on, I'll assume you're going solo - the same things will apply to wings anyway.
So first off, you want to carefully pick your fights. By that, I don't just mean going after Deadly or Elite NPCs, but also being the one to start the attack, not the other way around. So, don't give NPCs a reason to start firing on you: carry no cargo, have no live bounties. This is very important, as multiple hostiles joining in on a fight you're already in is a quick way to defeat.
Second, certain locations have much higher chances of high-ranked NPC spawns. High intensity combat zones would be the best, but I'd advise against going solo to those, as you will end up with multiple enemies targeting you. (You can avoid that for a time, but sooner or later it will happen.) So, I would recommend unauthorised installations (for example, Rahu 1), compromised nav beacons or perhaps High / Hazardous RES-es. (In this order.) The good thing with all these is that you get good NPC spawns, but aren't guaranteed to be fired at. As such, you can pick your fights. It's also important to note that NPC-s will fight among each other too (pirates pirating pirates, bounty hunters hunting criminals) which will also make your job easier.
Once you're in a good place, take out the small high-ranked ships first. Sure, they give less credits for bounties, but rather counter-intuitively, only combat rank matters, not what ship they are flying. As such, killing an Elite Eagle will give you as much rank progression as killing an Elite Anaconda. While being much easier to do. This is rather stupid, in my opinion, but it appears this is how things are. (You will receive roughly ten times as many credits for the Anaconda, of course.)
And that's pretty much it in a nutshell. Stick with high value targets as outlined above, and you should progress much faster than you currently do - and hopefully have fun too. I'd also recommend finding a ship that's more fun for you, and learning to use directional thrusters, FA-Off and fixed weapons, but you don't really need to do those last ones for NPCs.