How to beat Legendary. It is pretty tough but anyone can learn to beat it reliably given the time. These are just my opinions and I'm certain some people will of had alternative experiences.
Styles of play
To keep thing simple I only want to talk about the two broader ways of playing Fifa 15 coins here; A slow build up possession style, or a faster build up counter attacking style. I have played both styles extensively however I stick to the latter one myself now.
Slow build up possession
This is honestly the most reliable and safe way to beat the cpu. I have played like this and I'd often have clean sheets or only concede one whilst scoring 2 or 3. If you just want to win then definitely play this style.
I would suggest using 4-1-2-1-2 or 3-5-2 for these styles. You can use whatever formation you like but anything that packs out the midfield is perfect. It facilitates a lot of passing and allows you to drop back quickly if you lose the ball.
The reason this style works so well is that the cpu, much like a real player, will become more and more desperate to get possession the longer they're without it. You'll find them diving into more tackles and being dragged out of position more and more often as you keep it. Eventually this will lead to you being able to find a pass to open them up and score.
Faster build up counter-attacking
This style of play is far riskier against the CPU because they're deadly in possession. You may score more yourself, particularly if you're a decent dribbler, but they'll likely get more against you as well.
This style applies more towards formations like 4-3-3's, excluding the false 9, where you have quick wingers and players in the midfield who can get the ball pushed up quickly.
You're more likely to lose the ball playing this way and if you get frustrated easily you may find it annoying trying to get possession back off the CPU afterwards.
I play like this purely because I find it more entertaining. I play 4-3-3(4) and have both my wingers and CAM with instructions to stay forward and rely on my two CDM's and back four to deal with their attacks.
Important aspects to improve on
Passing - This has to be the obvious one here. If you're going to play a possession style you have to be able to pass well. There are plenty of guides out there and I don't really need to go into much detail here. Just retain the ball and don't get greedy or fancy till you're really experienced.
First touch - I think this is something that people may not consider as an aspect of their game to improve on a lot but it's really vital. The CPU players tackle like gods. They'll bend space and time to get that ball from your feet when you feel totally safe. Your first touches have to be precise. You need to know exactly where you're planning to go with a player or where you're going to pass next so you can turn into it. I would suggest playing as Barcelona vs Liverpool on legendary to get better at this with their current formations and players. Barca's players rely on nimble dribbling and creating space for their players to be effective and the Liverpool lads will be looking to close them down and knock them off the ball the whole game. I'm aware a lot of teams would do this but I think Liverpool are a good choice because they're somewhat challenging, but they wont crush you with their current lineup.
Strength - Learn how to use strong players because every single cpu player on legendary is basically Zlatan. You think thats Raheem Sterling? No. Zlatan disguised himself to fuck up your day. I'm over exaggerating it a bit but they will knock your boys off the ball very easily. Make sure to tap L2 repeatedly to get in front of players you know are weaker than you when shoulder to shoulder. Body blocking can also be really effective to lay off passes to wingers or your other striker. Try using PSG and playing the ball into Zlatan a lot to master how to use strength effectively.
Tips and tricks
Always change direction - The meaning of this is pretty simple. If you're sprinting alongside a defender and you just don't have enough room to get round him through pace then just stop and change direction. Do it with L1 then move somewhere else if you like or far better yet use L2+R2 briefly to go back on yourself then burst out of it with sprint. The reason this pretty much always works is that defenders are constantly trying to get into the "perfect" position that blocks you from running at the goal. They'll still be trying to reach that previous position even when you change direction. If that doesn't make sense then just go and do it in a game and you'll see what I mean.
Dont worry about specific players - Just because you're playing against Ronaldo or Messi dont be constantly thinking about how important it is to keep the ball from them. On WC/Legendary every cpu player is just as deadly. They've all had a massive stat increase and any of them can weave through your entire team solo. Just be aware of who their quick players are and don't leave them enough room to leave your defenders for dead.
Always always jockey to block a pass - I personally have no issue with the big stat increase the cpu gets on high difficulties. My biggest complaint is how they can always pass through you. I'm sure we've all experienced what happens when an opposing player is stood between your man is possession and the player you want to pass to. Instead of you passing it straight into the opposing player your man tries to put it into an area they aren't blocking, but that your target will reach and get the ball. So they'll basically kick it a foot to the left or right of the opposition player instead. Oftentimes this results in you losing possession and blaming the game for being a piece of shit. The cpu does not suffer from this problem however. If you're alongside one of their players and blocking the pass you know they want to make then they will just make it anyway and your guy will screw up blocking it. Unfortunately the AI knows when a ball will reach its target and it won't make the pass unless its certain the ball will get there. The only real way to improve your chances of blocking it is by jockeying just before the pass is made. It just means your player has his body facing the opposition so he's wider and can react quicker.
Draw fouls - If you're a good dribbler then don't be afraid of trying to get round their back four with some nimble footwork. You don't need to use skills, you can just take quick touches to get through them. The cpu's reaction to this is normally to go for a slide tackle. They do this very very often and will more than likely foul you. Free kicks are deadly in this game so you should be able to convert these fouls 90% of the time.
Don't accept their possession - I think in online play backing off when you lose the ball is the way to go. I don't feel the same way for offline. Once they're near your box the cpu will either pass it through you with ease or find space on the wings to cross it in which you deeply do not want. Even if you're excellent on defense you will make a mistake that will let them in eventually. Pressuring them is really important but don't be dumb about it. Don't drag players way out of position to do it. Use your CDM's to hunt down their ball carrier. Fight them the whole way down the pitch.
Pressuring their defenders - On World Class if you see an opportunity to bring two men to their left back or right back when they're in possession in such a way that will block a ground pass then do it. Normally the pressure will force them to lob it out for a throw in. On Legendary however you have to be far more experienced to know when this will work. They deal with pressure much better and will normally dribble to a point where they can get a pass away, which then leaves you with two players out of position.
Use fast Left and right backs - You have to be able to deal with them doubling up on you and making space down the wings to put in crosses. Make sure you're sprinting after them and blocking the cross as best you can.
