When the moment arrives that you know you'll be alright
I originially intended to put everything about Terra's battling here. I have the information saved and all I'd have to do is get it from my game data. The issue I had with that, is that I find it kind of boring in context with the other essays here. Instead, I decided it would be far better to explain how I use Terra in battle and why I think that's an effective way to play. Please keep in mind this is my play style and ideally, everyone should find what works best for them. I'm just giving some suggestions since Terra tends to be less magically inclined and a little slower than his counterparts Aqua and Ven.
I'll be honest, though, I like to play Terra as though he's a samurai. He's got the techs and the look for it. Depending on my mood, I have three different keyblades that I use for Terra. I use Ends of the Earth, Chaos Ripper, and Sweetstack. The first is his standard keyblade and you don't need to do anything special to get it (just go through his story). The Chaos Ripper is really good for physical attakcs which Terra already excels in. The Sweetstack requires you to collect all of the icea cream items and takes some tiem to do but is a very balanced (and manly) keyblade.
I keep Meteor Crash and Zantetsuken on Terra, they're extremely awesome and the latter looks very cool. I use mega flare for large groups of enemies, and usually one mega flare will take out almost everything (unless he has on the Chaos Ripper), one Curaga, and Transendence just has an awesome name. I've gotten through a lot of tough battles with these commands. I rarely if ever use the shotlock commands because it makes me nauseous. I change out the action commands if I need to (particularly Ice Slide for other elements). Mostly, I just hack and slash away. Terra benefits a lot from physical attack commands. Magnenga can also be very helpful to equip if you're having trouble getting enemies together (or if they fly). Otherwise, I just use things like Meteor Crash and Zantetsuken as finishing moves.
Terra's special finish command is Demolition. Unless I'm trying to get other finish commands, I keep this on him. I can't particularly say that I use it often. Battles usually end quickly, however, it can be particularly helpful in large groups or boss battles. The trick is controlling where the meteors and explosions land which can be done by moving the control stick.
Basically, I just play Terra as a warrior that can slash through anything and everything in his way. Keeping attack commands on generally keeps him on critical impact for his command styles, he gets a speed boost and can slash and hack away even more quickly. Playing with different commands and elements allows Terra to unlock different elemental command styles with Rockbreaker, and Dark Impulse being among his strongest. I find that I particularly enjoy Diamond Dust and Blade Charge though. For Diamond Dust, one must needs to do a combo and finish with ice magic. Blade Charge is one of my favorites though and can be triggered by using fire, ice, or strike commands. Since I keep strike commands on Terra's deck, this is often easiest to trigger. Although, I find it even easier to build off of diamond dust despite the fact that I don't keep blizzard on him.
Over all, I think the best thing to do is try melding commands and finding commands that you like. I find Terra the easiest to fight as because he's a little slower. My eyes aren't required to focus quickly and I don't have to worry about twirls and super speed (so I don't get nauseous). If you want a steady battler with great strength, Terra's your guy.