First off, I'm not a pro at this game. I just learned to play the game overtime, watched how others played, and got the experience I needed to win. I spent a total of 50 hours online, and 119 hours offline on this game until I got the achievement. That's a lot of time, I know. But for some people it might be easier depending on your history of fighting games.
Since the Rank system has already been explained here, I'll just jump in and give you some tips I used to get this achievement.
1. The difference between online and offline
I played this game offline with my brothers and friends. Then I jumped into online, and I found myself getting my ass kicked. Why? Because online suffers from lag (also because I was outmatched 60% of the time). You have to get used to it, and you will have to pay a price. You will notice that your combos will be dropping, your special moves won't execute, and blocking mix-ups will be near impossible. You have to understand that some characters are better for lag than others:
Hulk, Wolverine, Wesker, Vergil, Sentinel, Doom etc...
Some characters are incredible offline, but suffer offline due to execution:
Dante, C. Viper, Magneto, etc....
Not because Dante suffers from lag greatly doesn't mean that you can't be a decent player with him online. It just means that it will require a lot more work playing with Dante than, say, Hulk.
2. Learn the game
By learning the game, I mean learn everything. Learn how to play your characters, learn how OTHER characters play so you can develop a counter strategy, and most importantly, learn how to play intelligently. Understand that some characters are better played offensively, and others defensively. Learn to Snap Back, Team counter, and most importantly learn to call assists at the right time. Do not call an assist randomly as it can result in a "Happy Birthday" where both your characters will be caught in the same combo (1 free character dead!).
**Make sure to go to Youtube and look up tutorials, combo videos, check out the guides at Shoryuken, and spectate on lobbies.**
3. Be unpredictable
By unpredictable, I don't mean random. You have to play intelligently at all times. An example of a predictable player would be a Dante player who just teleports and uses the S attack. Or a Hulk player that abuses Gamma Charge, and his standing H attack. These can easily be punished. To play unpredictably is to play smart. Don't let your opponent get into your head. Change your method of approach, and your method of defense. Example: you play a character that teleports, and your opponent already predicts you always teleport behind him. Mix it up a little and teleport in front of your opponent. Your opponent won't expect it, thus you are playing intelligently.
4. Know your opponent.
When you play Ranked matches, you have some options:
ALWAYS, select Rank: Same. If you select "Any" chances are you will be paired up with an experienced player more often than a noob player. If you select Rank: Same, your chances of getting matched up with the same person over and over are fairly big since. If you keep getting matched up with a person that out classes you, then don't fight. No matter if that person insists, DO NOT fight. Losing will deduct rank points. If you go on a losing streak, you will rank down eventually. Just play against players you have never played against, players you know you are going to win against, and players you have a 50/50 chance of beating. Never fight if you have a higher chance of losing.
If you find a person that you are most likely to win against, fight them. If they won't learn that you can clearly out class them, it's their problem.
5. Don't get frustrated
This can be very hard for some people, including myself. Some days you will just not be in your zone. When you start going on a losing streak, the best thing you can do is stop playing. Don't even go to training mode because by then you will already be in a mental state where winning is rare. This affects your performance greatly, and leads to more loses because you will be desperate to win, and you won't be playing intelligently.
6. Never give up.
By never giving up, I mean don't rage quit. Ever. This won't affect your win rate, or win streaks. But it will greatly affect your rank points. You will lose a big amount of rank points if you D/C, even by accident. So not only "try" to not rage quit, just "don't" rage quit. You are better off taking a loss than rage quitting.
7. Win Rate
If you manage to have a Win rate of at least 50%, you have a solid chance of making it to 6th Rank. Slowly, but solid. if you go below, just keep on getting those win streaks. Win streaks earn you more rank points. Losing streaks deduct more rank points, so try to keep winning.
8. Stick vs Controller
I play with an arcade stick, but everything I can do on an arcade stick, I can do on a controller. It's all about preference.