Hugh will be handling all of the Team Plasma Grunts near the entrance himself, so head south and move through the doorway there. You’ll find yourself ambushed by two Grunts who’ll take you on from either side. Double Battle.
Double Battle: Team Plasma Grunt and Team Plasma Grunt
Male Grunt’s party
Female Grunt’s party
Once they’re beat, hop onto the teleportation pad to your south.