Pidgey and Rattata are more than enough. Even if one of those two were the only Normal-type Pokemon available in the game. the medal would be very easy to be earned.
2) Bird Keeper
The most common Pokemon’s secondary Type is Flying and also Spearow’s which is among the Top 5 for sure. A piece of cake.
3) Punk Girl
Ekans. This is your answer here. The 3rd most common Pokemon after Pidgey and Rattata. Also Nidoran (M) and Nidoran (F) are common, and let’s not forget that many other Pokemon have Poison as their secondary Type.
4) Ruin Maniac
Geodude’s second type is Ground, and they are very common to be found. Also Sandshrew is one of the Top 10 Pokemon found in the wild, and Diglett, Cubone and Rhyhorn if not Top 10, then they are Top 20. A very easy medal.
5) Black Belt
Mankey and Machop are on the Top 10 of the most common Pokemon to be found in the wild. Mankey maybe belongs to the Top 5.
Geodudes are very common and not hard to be caught as I said in #4. Also Rhyhorns which have Rock as their second type are kinda common (not as common as Geodude though), so go for it.
7) Bug Catcher
Caterpie and Weedle are relatively common to be found in the wild and they need just 12 candies in order to evolve. Also there’s a good chance to get one of those two from a 2 km egg.
As you might know, Water-type Pokemon are the majority, even more than Normal type Pokemon. So in that case you are lucky. But the bad thing is that if you live in a land, it seems that Water Pokemon are less common making it a little bit harder to get the medal. But if you live in a place near a beach or somewhere like that, then you are lucky.
Here the only one you must head for are the Growlithes. It’s way more common than Ponyta, Vulpix or Charmander and also for some reason, it’s easier to catch with a PokeBall, even if its CP is 600-800.
The good news here are, that Oddish, Bulbasaur and Bellsprout need only 25 candies in order to get to the next level. The bad news on other hand are, that those three are uncommon to be found in the wild. Let’s not forget Paras which has Grass as its second type.
The most basic Psychic Pokemon is Abra. It’s relatively common to be found and it only needs 25 candies to get to the next level. Drowzee is less common to be found though, and also Exeggcute has Psychic as its second type, but it’s less common than Abra as well. So head for Abras better.
Despite the fact Pikachu is the most popular Pokemon, and also the franchise’s mascot, it’s not as common as Pidgey or Rattata. Likewise, Magnemite and Voltorb aren’t common. Also the bad thing here is, that all three of them need 50 candies in order to evolve. But I’d suggest you to keep Magnemites in your inventory. Why? See in #16. Apart from Depot Agent, they are also helpful for the Rocker badge.
13) Fairy Tale Girl
Surprisingly Clefairy and Clefable aren’t Normal/Fairy but just Fairy unlike Jigglypuff/Wigglytuff. There’s also Mr. Mime who has Fairy as a secondary type. Clefairy and Jigglypuff aren’t uncommon to be found in the wild, and Mr. Mime isn’t so rare to be found, but not as common as the other two. It takes time to get Gold in this type of Pokemon, but it’s not the toughest one.
14) Hex Maniac
Gastly/Haunter/Gengar are the only Ghost-type Pokemon in the game, but fortunately Gastly isn’t rare to find –well at least in my area there are many Gastly-, and Gastly needs only 25 Gastly candies to be evolved into a Haunter. So keep in mind to not transfer your Gastly once you capture one, so you can evolve them into a Haunter later.
15) Dragon Tamer
Although the only Dragon Pokemon in the game are Dratini/Dragonair/Dragonite, and it’s rare to find a Dratini in the wild, the good thing is that Dratini’s egg (which needs 10 km in order to be hatched) gives you lots of Dratini candies, and Dratini needs only 25 Dratini candies in order to evolve into a Dragonair, making it less tougher than it seems.
16) Depot Agent
The only Steel Pokemon in the game are Magnemite and Magneton. Not rare, but not common yet. The best you can do is to keep multiple Magnemites in your inventory and evolve them into a Magneton once you reach 50 Magnemite candies each time.
This is the toughest among all. There are only 4 Ice-type Pokemon (apart from Articuno) as far as you know. Particularly, Jynx, Lapras, Cloyster and Dewgong. Jynx and Lapras can be found in the big eggs that need 10 km in order to be hatched, and you cannot tell If they are always in those eggs. Finding them in the wild is rare, and still if you find them they are not easy to be caught. So in order to get this medal faster you must keep multiple Shellder and Seal in your inventory (again Shellder and Seal aren’t so common as well, but not extremely rare), and evolve them into Cloyster and Dewgong respectively once you reach the required amount of candies they need. Of course in order to achieve this, you must expand your Pokemon Storage space, so you can keep multiple Shellder and Seals in your inventory. Once you capture a Shellder and a Seal, keep them and don’t transfer them.
And let’s not forget Dark type. Although Dark type attacks exist in Pokemon Go (such as Bite), strangely there are no Dark-Type Pokemon in the game, but normally they should have. Maybe Growlithe/Arcanine could have Dark as their secondary type. And if they did, I would have a Gold medal here, since I have captured more than 200 Growlithe so far!