Mega Salamence, kinda king of the current dragon megas? Will be outclassed, but easy to beat, easy to mega! Get yours for a good raid counter, PvP not so much..
As for what to use? Ice is double super effective, so you can search deep into your box for some chilly friends. Mamoswine makes it’s popular appearance, always a good fall back.
Go to the official counters page for a complete list including Shadows and Megas to see how your Pokemon perform.