If you have at least 10,000 gold, buy two Invincibilitys, two Rapid Fires, two Super Shots, and two Phoenixes. Use the rest of the money on any of those four to have some extras. Enter the battle and just hold A. You will not get hurt and the Boss will die in a few seconds.
Buy about eight or more Thunder Hammers. Go to any Boss, turn your Thunder Hammer on, and run up to it. Repeatedly hit it with the Thunder Hammer (tap A as quickly as possible). You will rarely get hurt, because when you are swinging the hammer you are invincible. You then will hit your target and swing again immediately after that. Your may need to buy more Thunder Hammers depending on the Boss. You will also get some gold and experience while defeating the Boss. This is also a good way to increase levels.
By lots of Super Shot and do not stop shooting at the Boss. Note: This will not work against the Lich and Spider Queen.
StvGam2.
Buy a lot of Thunder Hammers. When you reach the Boss' level, run directly up next to it and repeatedly attack. You will continuously attack without taking a lot of damage. The Boss should die without much of a fight. If you die before you kill it, buy more Hammers. This works much better if you are playing a multi-player game. Have both you and your teammate(s) run up and attack in the same manner. When you both/all have a full turbo meter, use your team attack. This will help kill the Boss faster. The Thunder Hammer trick works well on most of the weaker Bosses. The Boss of the Dream levels is too wise for this trick, and the final Boss is too strong. The Spider Queen will hit you frequently. In order to defeat her easily, start a multi-player game and designate someone as the punching bag. Whoever plays the punching bag should run up to the Queen and stand in front of her, causing her to get distracted by attacking only that person. This will work for awhile, then she will turn and try to get someone else. Make sure that the punching bag is always in her way. Buy a lot of invincible crystals for the punching bag character. Remember, if an item is timed, it will run out twice as fast in a Boss battle. The punching bag can attack the Queen and is recommended, especially since that means more damage faster. Also, do not forget to use your Team Attacks. They will deal a large sum of damage.
As a Jester or Archer, stand far from the Boss and fire repeated shots at it. When it gets ready to fire long range attacks, stop firing and move. It will miss and become vulnerable. Note: This will not work with the Lich or Spider Queen.
Vlaze.
This trick requires a lot of money. Go to the shop and buy a lot of Super Shots. Continuously fire at the Boss. The Boss will be attacking you, but you will not lose any health.
Akash Vasishtha.
To defeat a General or Golem without spending money or using an item, when you run into creates, go behind a wall or barrier of some sort that you can shoot over. Then, use your slow attack multiple times.