A good match against Brick (Oregon Pikachu player)
The Wario/Pikachu matchup is extremely fun. Both characters are very nimble, with Pikachu probably taking the #1 spot between the two. Pika's spotdodge is very good, and he can use it and then quickly throw out another move to disrupt your attacks. Many of his aerials beat out Wario's in the air, so Wario has to weave in and out between attacks, a bit difficult but by no means impossible.
My favorite moves to use against Pikachu are definitely Nair, bite and Jab. Nair's 4 frame execution is extremely fast and gives Wario some much needed move-speed in the matchup; Pikachu's moves are on average much faster than Wario's. Nair can also lead into combos at low and mid percents, allowing for bigger damage combos and thus bringing the lil mouse that much closer to death. Wario's Jab has a good amount of range and priority, also leading into a grab fairly consistently. Bite punishes Pikachu for shielding (and his shield is a B!&@# to get through, it's really freaking good). It is very difficult to punish a lot of Pikachu's low lag moves, but if you read a shield then bite will be able to punish and score a little bit of damage. The more damage pikachu takes, the closer he is to death.
Tires in this matchup are fairly risky, with good rewards if you play it right. Pikachu can easily take control of the tires, and can use them very well to his advantage. Keep tires out of Pika's grip if possible, thus Wario's best options I believe arise from Wario's standing item Dthrow. Hitting Pikachu in the right spot can put him in a position where Fsmash or Waft will connect, both devastating KO moves in their own respect. Dthrow glide toss can also lead into Fsmash, Fart or grab and can be very deadly. You can even link Wario's Nair with a Dthrow glidetoss into a hit! Now that is innovation, folks.
Oh and Pikachu's thunder jolts are extremely annoying for Wario. It's hard to punish Pikachu whenever he camps, but most of Wario's aerials will destroy the T-jolts in midair safely. Happy hunting!
+0 between the two. Pikachu's chaingrab on the edge of platforms can be deadly if you are caught in it, but if you are aware of it then there is no reason to attack the mouse of he is in position for it. If he is camping in the chaingrab spot, simply just pull out your bike and acquire some tires! This will definitely get him moving.