I bought this game 4 days ago, earlier I had 72 min of time playing it for free during event. I'm not a hunter, I didn't read any walkthroughs, any YT guides, any tips. Today I have character with almost 17 level, with nearly 100 kills, with 22 achievements (some rare), I don't use any special items. In truth I use only my Magnusson Lightning with bullets for 4-8 class animals. So, how did I do this, why I'm writing this guide, and why I won't show here any screenshots? - lets move forward.

A few days ago price of this game was reduced, so I bought it. Knowing myself I didn't buy any equipment DLC, but I did buy every hunting grounds DLC. So now I have 7 big maps, around 5x5 mile each of them, and what should I do? My answer is simple: I press "W". Every map contains many different areas, in one there will be less animals, in other more, somewhere animals will run in giant groups, and somewhere mostly solo. What I did from the start, was discovering every lookout point on two basic maps (Germany, and Lakes). When I did so, my map was covered in green points of interest, which I uncovered in shortest possible way (in most cases right after lookout point). For Layton Lakes map I did this way around two marathon lengths ( 9 hours ). And what did I have from this running?
-Fast travel to every region
-Around 7-8 k experience from "i" spots, lookout points and many other exp givers.
-Knowledge about territorial distribution of animals
-Many easy kills (more than 25), without slowing down, some animals simply bumped into me

Next I did the same for second map, Germany is actually better in terms of exp gain, around 5k exp more same way and same time length as first map.

A few tips & tricks that helped me in meantime:
-During discovery trips I placed small, heavy object on "W" button, many hours of pressing it was painful first day
-Animals have area of movement with borders, if you manage to press them into a corner of such area its like a massacre (easier done in mountains, and with group of animals)
-Some missions are easy and actually helpful, but I didn't focus on them. They don't give much rewards, and some are much longer than they look.
-I don't hit animals, which run in different direction than my own. Sometimes they can run away over half a mile from starting point.
-Of course I fall down from mountains, twice died this way, but straight line is best.

Added 07.04.2020
This section is a little more than gameplay basics, but its very corelated with previous one. So scoring is very important in this game, maybe not as something obligatory or part of missions, but still it gives pride and added value to your best kills. So imagine, that you've unlocked every hideout in every region on every map. If you own 7 maps, that gives around 100 hideouts, to which you can fast travel. Even better would be to imagine, that you looked around yourself uncovering maps, so now you know where are groups of bisons, mooses, deers or karibous.
Now its time to use binoculars. I don't know, if this ability is from the beggining, but after not much playing you can get info with your binoculars. For example you can see which animal in a group is the most difficult one. To see this you need to mark every of seen animals, and simply compare difficulty. Its easier to mark only males, because they will always give better scores, but sometimes its hard to say. So now you found your prey, you take out your most powerful gun for this class, take aim and shot. You have only one shot, try not to miss your chance to blow, afterwards whole group runs away, everything is a mess.
So to kill an animal with one shot, it's best to shot its chest (lungs, heart, backbone - whatever you'll choose), perhaps you'll hit head once in a while, I can tell you that its difficult to kill with headshot.
Now most expert part - how to see group of animals and stay unseen? I won't tell you, its not fun for me to walk in stealth for half an hour, so I just look through binoculars whenever I enter some large open space, perhaps near water there's bigger chance.
So I had five golden animals during 50 hours of playtime, when I uncovered maps. Using binoculars today I've got 4 golden in an hour. Getting a diamond is an achievement in which I'm not an expert.
And its easier to farm scores with animals from 4-8 class, simply they are less difficult, but in most cases large enough to hit easily. Class 9 is in my opinion for experts.
Probably lots of better guides on this thing, but I hope it'll help.

I wrote this guide, because I've read an opinion, that this game is not realistic and hard to master. Without a good plan this game is really tough, because it gives you freedom. So I began with knowing this game and its possibilities better. And taking photos of animals for mission is 10x times tougher than killing them, generally screenshots in this game are personal choice, because every place has nearly same quality - high.

And small spoiler: after improving my in-game skills I don't see much difference. Don't expect, that on higher levels you'll shot in straight line with thermovision. GL&HF