If you have guard I find that helps significantly (I am more of a the best offense is a defense kind of person, though...) and I always go through the game with a defense-heavy build.
What is it that is killing you? Random Heartless? Any ones in particular?
During the preliminary matches all of the heartless there have predictable, defendable attacks if you know how to watch for the patterns. If you have the MP for it (if you went with a mage build at the start) it can help to shoot out some spells at the big mobs (that aren't immune to magic) in the brief moment at the start when they are clumped up and then get yourself somewhere near an edge so you can keep an eye on all the enemies in front of you. Preventing getting sniped in the back is important.
Some random tips on specific heartless:
Red Nocturnes shoot fire, obviously, and fire is homing. Either guard/tech it, or try to attack it while it is charging the spell. Interrupting them is the easiest way to deal with them, I think. Dodge roll sometimes works, but often the spell will just chase after you. So in large mobs where these guys are present, I would try to go after them first because lots of fire can be super bad news. If you can't...last ditch effort hide behind Goofy, haha. Since you should have Blizzard at this point (you beat wonderland), if you have a bunch of them grouped up, spray them with that spell and it should reaaaally hurt.
Oppositely, I generally leave Blue Rhapsody until later because they have very obvious sound ques you can follow. If they jump back and you hear a charging sound (it's distinctive) you will then shortly after hear a shhhh sound as the blizzard spell traces along the ground towards you. You can easily jump over this. So even if you are fighting something else, if you hear them charging a spell, you can simply time your jump and leap over it without even seeing them on screen. (that is, if they are even targeting you). So if you have them in a mob they are more of an annoyance than an actual threat most of the time until they are dead. In the fight with a bunch of them, if you have the mp for it, throw out a few fire spells and instantly toast a couple of them to make managing things easier.
In the fight with the Large Body, I actually try to leave it alone as long as possible and clear the other things out first. That, or send Donald and Goofy at it while I clean up the other goons. Basically just keep an eye on him so you can jump over his shockwave attack if it does that near you. (for future reference, in fights with multiple large bodies, try to only attack one at a time so they don't all go into purple rage form all at the same time. That's just asking for trouble.) When it does that super rage charge in your direction at low hp, just guard it or stand behind Goofy. Even if an ally is dead, you can sometimes use their unconscious body for cover or to at least slow the Large Body down when it charges.
Soldiers have a pretty obvious tell when they are about to do their strong kick attack. They will kind of shudder and twitch then leap forward really fast. If you see any of them doing that twitchy-thing, just start dodge rolling if you can't tell where it's aimed. Generally that works. That, or you can try to interrupt them but if you get hit by that kick it can hurt pretty bad.
If you need to, don't be afraid to use Donald and Goofy and command them to attack things. Lots of people forget that this command even exists, but if you are locked on to an enemy and hit triangle it will prompt your party members to focus on that enemy. Sometimes this can be really really helpful if something is giving you trouble. You can send your party members to go whack at it and dps while you try to be a dodge tank away from it. (at least until they die. which they probably will).
Just make sure to bring some items with you, and if you have extra, just give a few to Donald and Goofy and adjust their AI so they don't immediately waste them all (but don't expect to see any of them back).
Otherwise, if it is the "final" match of the prelims that is giving you trouble (aka Spiky Boss Man), your best bet is to wait for the right windows to attack because he doesn't really stun easily and can interrupt your combos to get a hefty hit in. Often I find it better to wait for him to do the jumping move where he slams the sword into the ground (it leaves a little purple puddle) and then while he is drawing it back out hit him once or twice then dodge roll away before he can counter. If he is low on hp, fire-spam is always a cheap, safe way to win. Generally I try to stay at a fair distance in case he does his sonic rave move, then dodge roll away from that (he usually does it about 5 times in a row, but he can do it again or less but if you count the number of passes in your head it can help you get an idea on when he is going to stop approx.) then when he is finished roll in, smack him once or twice and roll back out. I play the irritating little thief-type character in this fight and it generally works out okay for me.
If you are really having a lot of trouble, you can always head off to deep jungle and try to beat that world first. Not sure if you are worried about spoilers, so I'll try to be vague just in case, but you get a magic spell after clearing deep jungle that might help if you find just fire, blizzard and thunder aren't cutting it.
