I believe the trick to Sonic players is not repeating their own patterns, because once you figure out Sonic's attacking patterns he's pretty much wrecked. Every Sonic player has his/her own way of using the special attacks to confuse and raise damage, so it's a matter of defending and finding his weaknesses, something all three pokemon can do.
Btw, if you figure out when a Sonic player is doing the homing attack, Ivy's Uair and Usmash are terrific punishment. Actually, for this match I like Ivy's options in the air from above and from below, which is good if the Sonic player abuses the spring.