Can absolutely confirm, that fight is just NOT a fair battle. Shadow Mons losing ALL weaknesses, AND having guaranteed super effective moves on EVERYTHING is NOT balanced. Both Shadows having extremely frustrating dual side hitting moves is just NOT fair or balanced.
I'd suggest, instead of making them lose EVERY weakness, make them have normal weaknesses, that way it's, y'know, balanced. That or lower their level a bit, or reduce their team sizes. That fight being a forced double, with 2 FULLY EVOLVED shadow mons?! Not fair or balanced
My main tip for speeding up levelling, is ADD MORE AUDINO IN THE LEVEL UP LADY
LIKE HOLY FUCK ONLY ONE AUDINO AT A TIME IS FUCKIN INSANE AND STUPID AS FUCK