Pikachu is one of my best characters. He's not very strong, and he has terrible armor, but he's really fast. His physical ground moves have good range, fire quickly, and are great for combos. His up-A smash is good for juggling and star KO's, and his down-A smash is exceptional for dealing damage and incapacitating assailants. His aerial moves are even better for combos than his ground moves. His aerial up-A move is his deadliest weapon. It does minimal damage, but it can spike your foe in all kinds of directions, making it almost like a meteor smash that launches foes obliquely or horizontally instead of straight down. It's much faster and easier to execute than any meteor smash, and it's consequently ideal for quickly canceling out an assailant's meteor smashes and other slower aerial attacks. Pikachu's other aerial techniques are also good for doing damage. His B moves are great. His up-B move performs up to two lightning fast, consecutive lunges; though very difficult to control, this is one of the best recovery moves because it allows you to choose almost any trajectory, if you can think fast enough. His down-B lightning bolt can be good for KO's, interception of aerial assailants, and juggling; I often use it as a final blow in a long combo of aerial attacks. His left/right-B move, which sends him flying horizontally, is very powerful when charged, but it leaves you vulnerable, and its trajectory is predictable, so it is best used in free-for alls or team battles. His B move is great for ranged attack and for stunning enemies.
Pikachu is not very good for ground-based assaults because he doesn't gather much momentum and because his running-A move is weak and easily intercepted. Fortunately, you can counter this problem by firing a thundershock (B) toward your foe and then sprinting after it. Since the speed of Pikachu's thundershock is very close to his running speed, either your projectile will stun the enemy, giving you just enough time to deliver a head butt, or the enemy will dodge the projectile and consequently be unable to dodge the rodent charging behind it. Use Pikachu for moving levels or those with platforms or multiple layers; on layered levels like Hyrule Temple, get on the lowest layer and zap everyone above with your down-B move. Specialize in aerial interception. If possible, play defense on a platform that assailants must access by jumping, and when they arrive, give them an aerial combo they'll never forget.