First, you need to unlock Piccolo, and unlock his 3rd and 4th tray slots. Then, Equip Piccolo with Special Beam Cannon lvl. 3, Destructive Wave and Light Grenade lvl. 3, Roast Pig(or something like that, it may be Roast Beef, etc.), Transform lvl. 2 (which should allow you to fuse with Nail and fuse with Kami), Hidden Ki lvl. 5(or 4, whichever is max), Ki Control! Lvl. 5(or 4, whichever is max), and Figher's Body lvl. 5, which is max. That will allow Piccolo to start a battle with full health, and when you get weak, Hold the X button and press down on the control pad or left analog stick twice to recover 5 bars of health. The other abilities allow you to stay in Ki Burn mode longer and allows you to consume less Ki while using your Ultimate attack. Since Special Beam Cannon does not allow a power struggle for the opponent to receive less damage, and being as fast as it is, it is literally impossible to dodge at close range, and very difficult to dodge at medium range. With Hidden Ki maxed, using Special Beam Cannon only consumes about 2 Ki bars instead of about 4 or 5. And when you are in Ki Burn mode, your ki doesn't decrease when using a ki-based attack, but instead over time. So when you have Ki Control! Max equipped, it allows you to stay in Ki Burn Mode for a good deal longer. So, before the battle starts, fly away from your opponent, charge your ki to a high level, then let your opponent get in close. When he/she is about to attack, use Transform! To stun them, then quickly enter max power mode, then use Special Beam Cannon. Then, Right when he gets up, he will roll to the side simultaneously. Right when the side step/roll is almost done, activate special beam cannon again. If both hit, then the opponent's fatigue bar will have been built up, and he will be temporarily stunned. Wait for him/her to COMPLETELY stand up, then use Special Beam Cannon a third time. Then, once he/she will get up, they will most likely Dragon Dash to the side. When the Side dash is almost over, then use special beam cannon a fourth time, and they should be dead. If you charged your ki high enough at the beginning of the battle, you could do this all in one Ki Burn Mode. I killed Kid Buu this way. Hope this Helps!!
First, you need to buy the Z difficulty in the black capsule section. Then, you need to have beaten Omega Shenron with Super Saiyan 4 Gogeta in the GT Saga. Then, continue doing Dragon Mission until you unlock new missions in the Majin Saga. One will have you play as Android 18 against Hercule. The missions is to let Hercule knock you out of bounds. Put the difficulty on Z and when the battle starts, rush into one of the outer corners of the arena. That way, when Hercule does a smash attack, it will knock you out of bounds. If you do it right, you will get a Z ranking and will receive 60,000 zeni. Also, if you manage to equip the WE GOTTA POWER!! Z item when it's maxed out at lvl. 5, then you can receive 120,000 zeni this way each time you do it. Note: try not to hurt Hercule or prolong the battle.
After you beat Omega Shenron in the GT Saga, continue unlocking new missions in story mode until you get to the Majin Saga. Keep unlocking new missions until you come across the World Tournament missions. One of them lets you play as Android #18. THe mission is to allow Hercule to knock you out of bounds. Unlock the highest difficulty (Z) by buying it in the Warrior's Room for about 10,000 to 30,000 zeni in the black capsule category. Then go back to the mission, set the difficulty to Z, then, when the battle starts, zoom over to the edge, and let him ring you out. Try not to hit him, and if he circles around you, use X and the analog stick to reposition yourself. Let him ring you out, and you will get 60,000 zeni for the Z ranking on the Z difficulty. If he tries using Present Bomb at the start of the battle, restart the mission. Doing this will allow you to get ringed out relatively easy. Repeat to get massive amounts of zeni.