i just want to say, i was messed up by that sonic stage =[=[.
ok, first off, shield more. if someone is in the air approaching you with an aerial, shield, then grab them after they hit you. the grab can usually lead into some more hits. if they try to approach straightforward, it is best if you spam a projectile or a disjointed hitbox to get them to run back and go for an aerial.
with G&w, stand backwards and spam his bair or fair, or maybe his nair. they wont come near you in fear of getting hit my a lagging disjointed hitbox. if they jump over your aerial and try to hit you from above, just shield grab.
for squirtle, you are definately best with him. you keep a good wall, as i noticed. just spam his aerials in place.
for ivysaur, spam his bair and his leaves, if they come near you shield grab.
for charizard, SPAM SPAM SPAM his fair, it is amazing for spacing. just play like you played with him, but shield grab a bit more.
i basically just gave you advice on how to camp, since it is very effective. but you shouldnt camp all the time, definately go in for some hits and mix it up. right now, you only have offense, which meant i could easily 4 stock you, which i did.