StrategyI have read that Aqua is one of the more challenging characters to play as in relation to Terra and Ven. But, I personally love playing as her. She is the only Kingdom Hearts character that I quickly bonded with in terms of battle style and mechanics. I'm certainly a lot better with her than Sora, anyway! In any case, if you find that you are struggling playing as Aqua or just like to read what tips and other tidbits someone else has to share regarding how they play as a character like I do, this is the page for you!
What I like most about Aqua is that she is relatively balanced between both strike moves and magic. She can theoretically go either way depending on what type of moves you prefer or have a combination of both. I personally tend to lean a bit more towards magic because her magic strength is the best out of the three characters. In general, I take advantage of her versatility and pick my plan of attack depending on the situation. If Aqua has to deal with a large group of enemies, I'll typically make use of magic to take them out quickly while being able to keep my distance. If there is only one or two enemies, I rely more on the attack command and may use a few strike commands if I want to do heavy damage. Shotlock commands are also a great way to deal a lot of damage if you have a single enemy that is relatively stationary right at the beginning. Aqua has a lot of variability in her reach depending on the attack. Therefore, it is pretty crucial to practice each move in order to get an idea of how close to the enemy you need to be in order to make contact. A lot of her spells can miss easily as well. I always lock-on whatever enemy I want to take out first for this reason. Also, though she is not as fast as Ven, Aqua is still a fairly quick and light character. Don't be afraid to jump around, glide and dodge in addition to/instead of trying to block.
Again, since Aqua can specialize in strike or magic attacks, the Keychains you use should reflect what types of moves you like to use most. I generally try to keep her as balanced as possible due to my use of both types of moves, but will lean more towards magic if necessary. Keychains such as Fairy Stars, Destiny's Embrace and Pixie Petal are good options for a command deck with a lot of magic. On the other hand, Keychains such as Treasure Trove and Mark of a Hero are better suited for a deck with strike commands. Since I do not own Final Mix and can't have Brightcrest, I always end the game using Stormfall due to it being the most powerful of her Keychains that is also well balanced.
I like to deal damage to multiple enemies at a time and therefore a good chunk of my command deck has moves that do just that. Some of my favorites are Time Splicer, Mine Shield, Magic Hour and Sonic Blade. Of course Aqua has to fight either one or only a handful of opponents as well and therefore it is necessary to have attacks that are suited for one enemy. My favorite single-enemy moves are Triple Blizzara, Thundaga Shot, Wishing Edge and Barrier Surge.
I think it is important to find a finish command that works well with your fighting style. Since I like to attack multiple enemies quickly, I am very happy using Teleport Spike as my finish. However, sometimes that can be frustrating when you only have one enemy left to fight. Therefore, you may like one of her other finish moves that attacks any enemies surrounding her such as the Magic Pulse series.
Action commands are also an important thing to consider as part of your strategy. Aqua can add elemental properties to Cartwheel and Air Slide (Firewheel and Ice Slide respectively) in order to do damage in battle. I would consider these a must as why would you not want to deal easy damage when moving around the screen? Not only do they do damage but they also will add to your command bar. Equipping reprisal commands are very useful as well. If you do not time Aqua's barrier properly, at least you are able to retaliate and deal a decent amount of damage in return.
Lastly, working with command styles and knowing what types of moves will lead you to the styles you like to use most is also very important. I personally love using Spellweaver, Ghost Drive and Sky Climber due to the speed and ability to glide around. Structure your command deck with attacks that will let you easily obtain the requirements for whichever styles you like best.