Assist Changing at the wrong time.
This is a big one cause this game is highly reliant on assist. There are some attacks you can't escape with an Assist Change unless its timed correctly. Others are just plain button miss-fire. Don't use it the moment the attack ends. It's pretty much useless at that point and you probably just locked your assist doing that. Don't do it while you're being sent flying either. Okay you wanna avoid the extra wall-rush damage but 9 times out of 10 its not worth avoiding the extra damage.
Missing or Just Plain Not Using Your Assist.
A lot of ppl are still used to the old Dissidia, which is very understandable. But not using your assist is pretty much letting your chances of an early victory out fly out the window. Its hard to get the timing of when to use assist but they need to be used. Even if its for a distraction purpose at least its getting used and granting you an opening. Its best to use your assist when the opponent has just been slammed to the wall for a sure fire hit. Also make use of the HP assist attacks as they may be the ones that give the chase options on some characters. Kuja and Aeris are not the only good assist available and don't work for everyone.
Not Taking Full Advantage of an EX Revenge
Its rare but this happens. Its happens cause a lot of use are used to the old method. You shouldn't just HP Attack the instant you have EX Revenge. Unless you're about to win. But otherwise get used to how much time you have to make up some crazy-will-never-work-otherwise combo.
Panicking During a Chase
I still have this problem. If you're thrown into a chase don't just immediately mash X. Yes the A.I. and a human player will probably smack you immediately but its better to watch then dodge. I'm not saying guess just watch closely. Pretty much all the time if its an HP attack you'll hear a sound or see a very long preparation animation. Just try and keep calm during the chase to avoid something fatal happening to you in a match.
HP Attack Spam
There is a true difference between spam and effective use. Spamming will leave you obviously open to attacks. May even see some get reflected back at you. I can't tell you how to not spam but avoid doing this.
Leaving Yourself Open During/After an HP Attack
This is hard to help as the recovery times some HP attacks are dismal. You must do all you can to avoid being left open. Cloud's Omnislash V5 is a big one. You may wanna block after the attack or just Dodge cause in some cases its going to take away to recover to a safe amount of Brave Points. Either get aggressive or get defensive. Try to read the next move but don't sit there.