A difficult battle scenario...
Latios VS Latias
While the former boasts a high Sp Atk, its counterpart compensates with promising Sp Def. In combination with 'Calm Mind', Latias' Special stats would become too high for the opponent to handle, thus causing severe ownage. However, this is assuming that Latios didn't also use 'Calm Mind'...
A battle between the Lati duo could take a lot of time, which is why I have no idea which Pokemon would emerge victorious. =P
If you see me posting, it's because I've been asked to. Which I guess would mean I'm taking requests.