There is no "Mystic Lake Cascino" in Minnesota. You likely mean Mystic Lake Casino Hotel, owned by the Shakopee Mdewakanton Sioux Community, located in Prior Lake (about 25 miles southwest of Minneapolis).
No direct public transit. MVTA Route 495 runs from Mystic Lake to Burnsville Transit Station (weekdays only), then transfer to Blue Line Light Rail (2 stops to Mall of America). Total ~1.5 hours.
If visiting both, do the Mall first (opens 10 AM), then drive to Mystic Lake for evening gaming/dining. Valleyfair (amusement park) is 5 minutes from Mystic Lake if you're staying overnight.