MagiusNecros
Gilgamesh Fan Annoyance
I can't say I have ever struggled against a Rob. He isn't even in the top 10 of characters that annoy me as Bowser.
Ganondorf's Wizard Drop Kick beats all of his projectiles. Bowser doesn't even need Dash Slash, but it makes it easier. Absolutely essential is learning the various ways you can pick up a projectile item thrown by your opponent.
To be fair I've never fought ROB with the Customs on before so things will probably be different.