legendary souls isn't exactly a good way to measure your skill, as the computer seems to take predictive measures in that mode. and when a computer is predicting you, you're really screwed, no matter who you are. that mode is considered a joke due to how insane it is.
i'd say, play around online in player match. watch the other players while waiting for your turn, and when you see good players, find a way to play against them. if you don't have much trouble from them, then i'd say you're ready to at least attempt competetive play.
legendary souls is no more than a simple AI with the ability to command read, so treating it like a human-player and trying to implore mixups etc isn't the wisest of strategies...it can however be cheesed relatively easily, so its not as hard as people make it out be
No, dude. Learning the basics, the system and the combos is easy. The hardest part is pretty much always going to be your opponent. Just train with a character you like, take a bunch of online matches or local matches with friends (or better, join a local fighting game community if you have one nearby) and you will see that it will pay of when you go on some tournaments or w/e. But never rely on learning anything from playing with CPU's because that will not help at all.
Note that tournaments is a really good way of learning too.