Greetings mayor,
this achievement guide will help you get all achievements in Cities:Skylines II. The following achievements are listed in an order that is based on the percentage of other mayors who have already unlocked the respective achievement but I assure you this is only temporary.

So let's get started, shall we?

Build city with residential, commercial and industrial zones, water, and electricity.

This first one is self-explainatory. You will get this as your first achievement garuanteed.

Have buildings from all four zone types in a single city.

Reach Milestone 5 "Grand Village" and place down some office zones. Together with the residential, commercial and industrial buildings you'll already have the achievement should pop once the first office building grows.

Reach enough milestones to unlock all city services in a single city.

Simply reach milestone 5 "Tiny Town".

Have at least 1000 citizens and an average happiness rating of 75%.

Make sure to cater to all your citizen's need by placing schools, fire houses, police stations, hospitals, and parks to make them happy. Make sure you haven't set your taxes too high but other than that it's actually rather difficult to make your citizens unhappy.

Unlock 50 map tiles in a single city.

Unlock 150 map tiles in a single city.

You will get both of these as your city grows. By milestone 8 "Big Town" you'll have more than 50 "expansion permits" and by milestone 12 or 13 you'll have reached 150. Simply buy the respective amount of tiles as you need more space for your city and you'll get the achievement. Please note that your city starts out with 9 tiles already unlocked so you need to buy exactly 41 tiles for "The Explorer" to unlock. Be careful to buy the exact amount of tiles, otherwise the achievement might not unlock. Thank you to Space Shadow for pointing that out.

Plant 100 trees or bushes with the landscaping tool in a single city.

Easy. Go into the landscaping menu (the shovel icon) and select any plant from the vegetation tab but select the "Place multiple" tool option on the left. Then just paint over any plot of land and the achievement will pop in no time.

Assign a policy to a city district.

Use the district tool and make a district anywhere in your city. Then select that district and enable any of the policies. I recommend doing this achievement together with Happy to Be of Service and Wide Variety.

Create 10 different districts in a single city and assign policies to each.

This one is just Executive Decision times ten. However, unlocking isn't as straightforward as Executive Decision. Each of the 10 district must have a different combination of policies for it to count, so just assign 10 unique combinations. I recommend doing this achievement together with Happy to Be of Service.

Create a city district and assign a city service to that district.

Go for this one together with Executive Decision and Wide Variety. Once you have created a district, simply click on any city service building like a clinic or school, click on "Select operating district" and select any district. Done!

Have a total loan of at least 200,000 in a single city.

Once you reach milestone 3 "Large Village", you'll be able to take out a loan of ¢200,000. Just take the full sum and accept the loan then pay it back after you got the achievement.

Have a city that produces electricity by only using renewable energy sources.

Start out building any power plant and once you unlock and place any form of advanced renewable power generation (geothermal, hydroelectric, solar) the achievement will pop after any and all polluting types of power plants are turned off. Thank you to Eltoron for noting that any advanced renewable power plant makes the achievement pop as long as no other polluting power plant are active.

Have a look at each individual info view panel.

You can get this right after Royal Flush by simply looking at every single info view after reaching milestone 5 "Tiny Town".

Use the photo mode to take a screenshot.

Open the photo mode by clicking on the camera button in the bottom right corner and pressing the "Take Photo" button. Easy.

Build 5 signature buildings in a single city.

Build 10 signature buildings in a single city.

Just build the respective amount of unique buildings. This includes both the unlockable buildings from the "Signature Buildings" menu as well as the landmarks from the "Parks & Recreation" menu.

Have a total of 10,000 tourist visits in a single city.

You will probably get this one passively as any decently decorated city will attract tourists but placing additional parks or landmarks will speed this up.

Experience all four seasons by building a city in a climate with a snowy winter.

Simply play the same city for one in-game year and it'll pop. (BTW, every "day" in C:S II is one month of the year.)

Reach a population of 100,000.

Grow your city until you have 100,000 citizens. That's it.

Have 6,000 citizens treated at medical clinics or hospitals in a single city.

You will passively gain progress on this one as soon as you have at least a clinic in your city. Don't try to speed up the process by pumping polluted water into people's homes, you'll lose more citizens that way. I got it when my city was at around 55,000 inhabitants.

Have 5 city policies active simultaneously.

Reaching milestone 10 "Small City" unlocks the last city-wide policies (accessible through the "City Information" panel, the button to the left of the demand bars). Simply enable all of them and you'll get the achievement.

Have 20 active transport lines. These can be passenger or cargo lines or any mix of the two.

Have 50 active transport lines. These can be passenger or cargo lines or any mix of the two.

This achievement can be easily cheesed by placing down two bus stops on a piece of road and using the bus line tool to create bus lines between the two bus stops. Once you unlock Spiderwebbing just right click on the nodes to remove the bus lines again.

Have at least 1000 citizens and a city attractiveness rating of 90.

Anything that has the "Indoor/Outdoor Recreation" attribute such as parks, landmarks and even the sports field upgrade for high schools add to the attractiveness of your city. So build more of those. Check the tourism info view to see your city's overall attractiveness.

Bulldoze a total of 1000 buildings.

This can easily be done by, for example, demolishing an industry building when there's a lot of demand for industry. A construction site for a new one will immediately pop up so you can just keep bulldozing buildings until you get the achievement. You can manually click or just use an auto-clicker.

Build a city where at least 20% of the population has a university level of education.

This will take a bit longer. You will have to have enough capacity for elementary schools, high schools, colleges, and universities so that every citizen that could pursue the goal of a university education can actually pursue it. Check the education info view to see the progress by looking at the "highly educated" portion of your population. The achievement will actually unlock at 15%, not 20% as its description states.

Reach milestone 20.

Pretty straightforward. Keep growing your city and you'll get it eventually.

Build 10 parks in a single city.

This one is very easy. Simply build 10 small parks to unlock this achievement. Should the achievement not make any progress, try starting a new city and quickly build it up to Milestone 4. Now you can place those 10 parks.

Use the editor to make a map.

This one's rather easy. Open the Editor and and click on "Map" in the top-left corner. Enter a name for your map and click on "Save Map" at the bottom, give it a file name and save. Done! (Of course you can also first shape the map to your liking but I will leave that at your discretion.)

Use the editor to make an asset of any other type than map.

Currently, the easiest and possibly only way to get this achievement is to go into the editor and place an object (via the house icon at the bottom). Then select the object from the "Objects" section on the left, click "Duplicate Template" on the bottom right to duplicate the object. Now you just have to place the duplicated object and click the save icon next to the copy that has now appeared in the Objects list on the left. This should make the achievement pop.

Many thanks to Hiltuska for the tip.

Have at least 1000 citizens and an average happiness rating of 25%.

Grow your city until you have access to low rent housing.
Zone the low rent housing downwind from a source of air pollution like a coal power plant or incinerator plant. (By zoning an alternating pattern of industry and residential you might be able to add the High ground pollution debuff as well.)
Make sure you relocate all of your population into the air pollution zone.
Make the zones only 2 tile deep so you can fit as many people as possible into the space you have.
Place a water pump that collects clean water and one that's placed right next to your sewer outlets but turn that one off for now.
Do not place any mail offices, parks, police or fire stations, clinics, or schools.
Once your population has reached at least ~2500, turn on the polluted water pump and turn off your sewer outlets.
When the polluted water has reached the people's homes, pause the game and reduce all service budgets to 50%, increase power and water fees to 200%, and set the taxes for residents to 30%.
Unpause your game and hope that the happiness falls below 25% before the population drops below 1000.
All of this together should result in the following debuffs: (My debuff values are in parenthesis)

• Polluted water (-15)
  
• High crime (-15)
  
• High taxes (-12)
  
• High water fees (-10)
  
• High electricity fees (-10)
  
• Polluted air (-10)
  
• Small homes (-5)
  
• Lack of entertainment (-4)

The only buffs to happiness I had at this point were Wealth (+9) and Reliable mail service (+6).
Thank you to GGreenie for the pointers.

Follow a citizen's lifepath from childhood to old age.

Find yourself some child citizens and add them to your list of followed citizens. Now you will need to wait for them to become seniors and not die before that. This should take around 7-8 in-game years, more the younger the child was when you started following their lifepath. (Should you have the developer mode on, enable "Show developer info" in the Gameplay tab and check their age that way. Citizens will turn from adults to seniors at the age of 85.)

Build any airport.

Very simple. Build an airport.

Experience a forest fire.

You don't need to do anything for this achievement. It will unlock automatically once a forest fire occurs.

Experience a tornado.

Another passive achievement. Just wait for a tornado to occur.

Experience a hailstorm.

And another one. Just wait for a hailstorm, though my guess is that this one can only unlock on maps that actually experience winter.

Unlock each building available in the base game.

To unlock this you must not only unlock all the different service buildings through the development tree but also unlock all signature buildings both from the European and the North American style. Luckily, industry and office signature buildings are shared between the two styles.

Build all unique city service buildings in a single city.

As the description states you must build one of each service city building. This does not include signature buildings. It's best to grow your city until milestone 20 and having ~ 500 million ¢ on hand before starting to build everything. Find yourself a large area of land and plop down everything there. The achievement will pop once all are placed.

This is all for now but I'll update the guide as things unfold. Until next time, mayor.