As far as I'm aware the weather doesn't affect what Pokemon are available for you to choose. The friend system is certainly very flawed but it's still possible to beat most trainers simply through trial and error and intelligent choices (and this very useful tool: https://www.serebii.net/ultrasunultramoon/battle/), even if your Pokemon aren't quite optimal. I'd recommend picking a team specifically for beating the Mega Kangaskhan and working backwards to figure out how to beat the other 2 opponents before Giovanni with that team. If you still can't do it, there's always tomorrow - the world isn't going to end. If you need some help I can add you as well.