Do you want a Reshiram? For PvE? Yes, it’s a good counter, worth getting, use the same one for PvP! Its exclusive move Glaciate just makes it better.

What should you use? Some of these guys are harder to find, Garchomp was a community day, if you missed it, ask a friend, it doesn’t need an elite tm or anything. You could also get a Dialga from someone who has raided, and do a raid, then use Zekron and Reshiram against themselves!