A recent update to the game introduced two rather notorious achievements, granted for reaching the 10th and 21st island respectively in a single run. Looking at the game’s discussions, people aren’t too happy about the amount of time this takes due to the randomness of the generated islands, a single failed one of which will cause your entire run to be reset.

This guide is mostly for those of you who can’t justify grinding this game for days on end to get these achievements and want to take a shortcut without outright cheating. However, there's also a section dedicated to some general basic tips on how to improve at the game, which you'll need to do either way.

The way you can use to make these achievements less annoying is essentially by abusing the save system, thus circumventing the game’s perma-death mechanic. However, there’s a bit more to make this actually efficient, as outlined in the section after this.

First of, when using windows, the save data is currently stored in your Appdata folder under:
The Appdata folder can be accessed by pressing the "Windows"-key and "R" at the same time, typing "appdata" in the promp that opens and clicking "OK".

 
I can’t seem to find up to date information on where the files are located on Linux and MacOS; PCGamingWiki suggests
for the former and
for the latter. Given that the "Grizzly Games"-Folder was changed to "Coatsink" on Windows, I’d assume the same to be the case on the other platforms, but I can't verify this. You’ll have to look around for the files yourself.

The file relevant for this guide is named "TheSaveV06.island" and contains your current active run. Simply make a copy of it, move said copy back into the save folder and reload the game whenever you want to reset to this previous state. This won't affect your run's progress towards the achievements.

Now that you know how to return to a previous state, it's time to learn a bit about the mechanics of the island generation and how you can use these to make your run even easier.

First of all, how many points you finish an island with has no impact on the points required for the next one. This means that to be as time-efficient as possible, you should move to the next island as soon as it becomes available. Playing for as many points as possible will merely improve your highscore, if you care about that.

Secondly, the seed used to generate the next island is set the moment you click the button to move to it. Thus, getting a "bad" island isn't predetermined for your run and can be remedied using the method outlined above.

 
Instead of copying your save at random points, make sure to copy it just before clicking the button to move to the next island. If the game give's you an island you can't manage to complete, simply restore your copied save file, reload your game and click the "next"-button anew. The game will generate an entirely new island, which should hopefully be more favourable. This can be repeated however often you want.

Follow these steps and you should be able to get both achievements with much less time investment and frustration.

I can offer a few basic tips should you want to go for the challenge without this trick - even with it, you'll need to have a good grasp of the game's mechanics to get through all islands:

1. Take some time to plan out the layout of your island before putting down any buildings at all - i.e. where to grow cities, where to leave space for large structures etc.
   
2. Keep planning ahead throughout the game. Many late game structures in particular can give a bunch of points but will require rather peculiar placement and you should actively prepare spaces for them. Also, keep in mind that losing a couple of points on one building in order to set up a higher score for an upcoming one can be very beneficial.
   
3. Learn which buildings should go near each other and which need to be placed apart. Also, pay attention to the orders in which you choose new building packs and place your available structures. There are a couple good guides available on these topics.
   
4. Take breaks between islands. Going through the same gameplay loop over and over again can get annoying easily, especially with later islands taking quite a while to complete. So consider taking things one island at a time to avoid tedium and frustration.

Even with perfect strategy, you'll still have to get lucky to a certain extend. An island with little space or lacking features can easily screw even the best strategy. Take it from me - someone who has decided to go for the 21 islands without the save game trick.

I hope I managed to make everything clear; feel free to comment if there's anything you need clarification on.

Also, consider rating the guide and/or dropping an award if it helped you. I’d very much appreciate it.