From the Main Menu, select 'Online Battle', then choose 'Mini-game Frenzy'.
Create match, 'Open' as the session type.
Change players 3 and 4 to 'COM-Easy' by pressing X and making the selection by pressing A.
Invite a friend/boosting partner to the room.
Press RB to choose the 'Aqua Blast' mini game and start.
Alternate wins between 1P and 2P, and use 'Return to Lobby' each time to setup the next match. Players in the sessions already, will remain there.
The first match won, will net you the 'The First Victory' achievement.