Provided that the unbeatable computer cant predict what items are going to spawn and at what moment...
1. Start a 300% match against the "Unbeatable Computer"
2. Turn bombs and smart bombs on and put items on "High"
3. Choose a stage like battlefield with no safe spots
4. Repeatedly hit the computer with projectiles, forcing it to dodge and perfect shield or rush you
There are two scenarios at this point. One where the computer simply stalls by perfect shielding your attacks, and one where the computer decides to get in close and counter attack. Here's what you do in each situation.
If the computer rushes:
1. The computer cant read what items will fall and when. Just stand there and taunt. If your character has a grab with high lag like Lucas, Yoshi or Samus use that. Present a nice vulerable target.
2. Pray that bomb or exploding capsule spawns at the exact moment the computer attacks.
3. Jump for joy when the comp gets haxed
If the Computer Stalls
1. Keep shooting until a smart bomb falls at the computer's feet.
2. Hit the smart bomb with a projectile. Odds are, the computer will be suffering a several milliseconds of end lag from the spot dodge or sheild and won't be able to avoid the explosion.
3. Make Unbeatable computer toast.
You can also avoid all of that by making a stage entirely out of falling platforms. Unless the computer can hack the game and fly, Its going to die. Just make sure you have more than one block to spawn on. If you spawn on the higher block you win.