Pikachu/Pichu mains will throw lots of thunder jolts (was that the name? anyway that thing that bounces on the stage) so a good way to fight them is drop the bomb on the stage to stop/force them to not use it, they can grab the bomb and throw it away too, so you can blow it if they try get close to it. Watch out for aerial attacks as they are multi-hit moves, so shield them until are finished. Link's NAir, UTilt and UpB help too as a get-off-me options. Link's Hylian shield doesn't help much blocking Pikachu's projectiles because they bounce and Link still could get hit. Be careful if you're high up and Pikachu is below you, you know, the Thunder special attack.