On Super Smash Bros. Melee, you could unlock the secret characters and stages by either beating Arcade or Classic mode at specific difficulties with specific characters, or you could grind multiplayer Versus matches. There's an AI error/glitch with the Level 9 Ness specifically on the stage Jungle Japes where if you stand perfectly still when the match starts, the Ness will attempt jumping to you, but will always fall into the water below and die immediately. You could use this with the One Stock (life) setting to grind Versus matches quickly to unlock all stages/characters.
On Super Smash Bros. Melee, you could unlock the secret characters and stages by either beating Arcade or Classic mode at specific difficulties with specific characters, or you could grind multiplayer Versus matches. There's an AI error/glitch with the Level 9 Ness specifically on the stage Jungle Japes where if you stand perfectly still when the match starts, the Ness will attempt jumping to you, but will always fall into the water below and die immediately. You could use this with the One Stock (life) setting to grind Versus matches quickly to unlock all stages/characters.