I just learned that about pet battles this weekend too. Tried doing a few but it's kind of a pain. Level 1 pets are shit, it's like a pokemon battle where both sides only know tackle and you are just like, well I hope I win. That said, you can win them, but when I won 2 of my pets died so only the last pet got xp, went from level 1 to level 4. When I tried another pet battle, I guess the scaling is based on your highest level pet, so I was against 3 level 4 pets to my 2 level 1 pets and 1 level 4. I had to forfeit and put in a new level 1 pet, then when I won that my pet levelled to 4 again...
Anyway, as you can see it isn't a big issue, they are still winnable and it's better than not being able to do them at all, but they can still be a bit annoying.
Oh, also you need 3 pets to win. Idk maybe there is some baller level 1 pet that I don't have who can 1 shot every pet out there, but my guys were getting trucked. To unlock the other 2 slots you need to get a pet to level 5 to get the 2 achievements. I just went to some low level pet zone and did that, only takes 10 or 15 minutes. However, when you get the achievement it doesn't seem to unlock the slots until you do a /reload for some reason.