I did the Lumbly Bridge portal after the arena myself and it dropped the recipe for Su's ultimate weapon and Dark Matter which is needed for Lex's ultimate weapon. I did a second portal West of Paradise Isle and it dropped almost everything minus the recipe (so I'm not sure if the first one you defeat drops it or if it's exclusively the Lumbly Bridge portal). I'm assuming the last portal I could find northeast of Regull Port will be the same.