To defeat enemies easily, use Gohan, Goku, Trunks, or Vegeta. First, turn Super Saiyan. Then, use Kamehameha, Burning attack, or Big Bang attack. This will not work on androids, except for Cell. Continuing from the energy attack, hit them with a series of punches and kicks. End it with Flurry punch, Super kick, Sword Blast, or Two Handed Smash. For Vegeta you should try using Energy Punch and Big Bang. Alternately, do the Energy attack then launch two regular blasts and two punches. Then, turn Super Saiyan and repeat.
When at the Capsule Corporation training arena (behind the Capsule Corporation building), hide behind a pole. Opponents will walk up to the pole and keep walking into it. You can punch through the pole to hit and kill them, but they cannot hit back. Note: This does not always work, and goes faster the higher level you are.