Pokemon Medals are separated into two categories, as you can see in your Pokemon GO profile as well. The first part are the trainer medals which are mostly based on Pokemon GO environment than Pokemon themselves and the second part is based exclusively on Pokemon Types.
(From the easiest to the hardest)
It doesn’t take too much to evolve 200 Pokemon. Especially Pokemon like Pidgey, Caterpie and Weedle who need only 12 Candies each, in order to evolve.
It’s very quick to get the first 50 Pokemon. And it’s only a matter of time to evolve (most of those) till you reach 100.
If you live in a city then you are lucky since you have many Pokestops available. But if you live in the countryside, then this medal would be really much harder to earn. Anyway, go to the nearest park. Parks have lots of Pokestops.
This medal requires only some time. It doesn’t matter which Pokemon you will capture, as long as you capture one.
Again this requires time, but it’s much easier thanks to the glitch, where you can walk from your home without having to go out in order to walk.
This is not much different than Jogger. But obviously it takes more time than the former. You can make it faster by keeping multiple eggs in Incubators, although it’s more suggested to keep the Incubators for the 10 km eggs.
7) Battle Girl
If you have a friend of yours, who’s a different team, then it’d be a good idea to go to a gym together, and sign some weak Pokemon in order to let him/her defeat you. Then he/she will do the same and…well you got it how it works. That way you’ll reach your goal sooner.
8. Ace Trainer
This one requires lots of patience. When you sign a Pokemon at the gym, keep in mind to train it as much as you can, before they kick it out, so you can reach your goal.
9) Tiny Rattata
This and the following two medals, are more like a separate category, even though in your profile page they are placed along with the 8 Trainer Medals. This one is the easiest among the three, but of course not easy. Sure Rattata is the second most common Pokemon...but chances to get an XS one are 1/10. So it takes lots of time and you need to capture around 3000 Rattata in total, to reach 300 XS ones.
10) Pikachu Fan
It’s not so easy to reach 200 Electric Pokemon as it is. So gathering 300 Pikachu as they are, is tough as nails. Definitely the second most difficult medal after Fisherman.
Ah this…Magikarp isn’t common in the wild, but it’s more common to be hatched from 2 km eggs. Imagine how many years you need in order to not only try to find a Magikarp, but an XL one. If you want to earn this medal, then you have to play this game for a decade at least. Seriously.