There's really no short way to put this. By no means is this a perfect suggestion, but it's what I tend to do, and it's relatively simple and easy, especially if you've got a partner to play with.
If you're by yourself, this advice is even more important, since you don't have any friends to fall back on if you fail to increase a certain stat to where it's of any use.
When you're starting off the game, play through the first few levels (up to the Barbarian Boss) as many times as possible so that you can get to level 10-ish. This will net you 18 stat increases. I'd say to put around 5 of these into magic, 5 into strength, and 3 into defense.
Feel free to go more into defense than one or the other, but you need to know (especially if you're flying solo), that having a sizable amount of both strength and magic will make your life a whole lot easier.
As you progress further, make sure to increase your strength, magic, and defense in increment. By the time you're around the last few levels (icy areas), try to have your magic stats increased all the way to the last colored bar (this is about 4/5 of the entire stat bar, being 20 individual slots), this way you can use the RT+A magic attack, which gives you a good boost for when you're fighting the final boss. Plus at this point, your RT+Y and +B attacks are of solid strength.
Also have your strength at around 2/3 completion. You should be attacking somewhere in the mid-20s at this point, which is a good place to be.
Have defense at around the halfway mark (or a bit over if you'd rather feel safer). Especially around the end of the game, you're going to be getting hit more and more, so you need to protect against it.
That's it for stats. Now with weapons and animal orbs, pick whatever suits the moment. For the majority of the game, I tend to stick with the Giraffey animal orb (found in the Desert level by digging around 2/3 of the way into the level), which boosts your EXP by around 10% (or so I've heard). During boss fights, I prefer to use animals that boost your stats like Chicken, Beholder, etc. Even if they only boost a couple points per stat, it all helps in the end.
The weapon you choose also changes things entirely. If you can manage to get it, go with the Wooden Club (what the panda bears have). This increases your strength and defense by 5 (though it takes some agility too), plus it has a critical hit bonus, which can multiply your damage up to around 4x. With a weapon like this, you can afford to lean back some on strength and defense and contribute more stat points into magic if you so wish.
If you can't manage to get that weapon, look around online for lists of weapons and find one with stat increases and abilities that suit your needs.
The fighting style I primarily use is jumping into the air and spamming the Y button. In large groups of enemies, this can be really useful. What will happen is you'll jump into the air and start attacking, but due to the amount of enemies being hit, you'll stay in the air while still hitting them. Control the direction you're moving in, and you'll take them down in no time.
This also helps out immensely in some boss fights (Ice King, Corn Boss, etc.)
In boss fights, the weakness is always out in the open, whether it be simply beating them to death or hitting certain spots. Just learn to watch your enemy's attack patterns, and you'll be able to take down anything in your way after a couple tries.
Whenever you get a chance to move around on the map, take this opportunity to max out your potions (5). When you're playing with others, you don't need to worry about using these since they're automatically activated when you run out of health, but for those of you that are playing alone, you'll have to use them manually via the B button (cycle through your items with the LB and RB buttons).
If you ever find yourself unable to beat a certain spot in the game, back up a few levels and replay parts of the game to level up more. Chances are, you either don't know exactly how to beat the boss, or you could use more HP and defense.
Other than that, check out other guides online to find out how to beat certain bosses or anything else.
Useful links:
Weapons Guide -
http://www.xbox360achievements.org/forum/showthread.php?t=89...Animal Orb Locations -
http://www.xbox360achievements.org/forum/showthread.php?t=88...Castle Crashers Wiki Page -
http://castlecrashers.wikia.com/wiki/Castle_Crashers_Wiki