this in a different section of TA. i don't know why this isn't in a guide so here it is
Hints & tips
First of all, Skull Island is currently not co-op! You will need to do this solo, but luckily the achievements aren't too bad.
Time is, again, of the essence. Difficulty will increase over time, so getting things done early on will help you.
Your focus is economic growth/sustainability and destroying 4 portals. It does not matter which portal you destroy, but I would recommend doing 2-2 or 3-1 depending how your world has spawned. I went for a 3-1 strategy so I could max. upgrade a tower at a good spot after destroying just one tower on the right side. That way I knew this side should be save and I could focus on the other.
There is a peaceful night after each blood moon, the perfect time to expand. Be careful, the end of a blood moon is not always the start of a new day.
Don't forget the Hourglass Shrine. You need money for it to avoid that volcano in the background erupting. Letting it run out will force you to restart. One level is one day/night cycle. So keep an eye on it if you don't have enough cash on you to keep it at least half way full.
There are a total of 4 coin chests. Try to find them during the first few days to give your Kingdom a financial boost.
The Skull Island Run
Start the game and select 'Challenge Islands' when you have loaded in. Choose the first one top left: Skull Island!
You might want to start over a couple times to find the "perfect" set up where you spawn near a chest, and where a spawn camp and the Hourglass Shrine are close to your camp. Because you will start on the Griffon with 0 coins. Scout the island for one treasure chests and make your way to your camp. The points below are a good indication if you want to destroy the 4 portals before day 30. Don't worry if it takes you a little longer, but you should be done before winter is coming (day 49). When following the indications below, I assume you have completed the base game and you know what to do.
Day/Night 1
-> Get 1-2 builders.
-> Get 1-4 archers.
-> Build basic walls and upgrade them once (if possible).
-> Use your Griffon ability to help your archers.
Day/Night 2 - 5
-> Keep recruiting - builders and archers.
-> Keep cutting down trees as far as you can and let the grass expand.
-> Scout for farm spots and expand to the nearest one till you can build one.
-> Scout for Hourglass Shrine if you have not found it yet. No need to put coins in yet. If the Hourglass Shrine is really far away you should restart.
-> Upgrade your camp and walls as far as you can. You should hit Stone Fortifications at least at the end of night 5. It is possible to use the 'free' walls when upgrading the camp at certain points.
-> At the end of the wave of night 5 it is time to fill up the Hourglass Shrine. It will cost 3 coins per level.
Day/Night 6 - 15
-> Don't forget to put coins in the Hourglass Shrine.
-> Keep recruiting - mostly archers from this point out with a squire in the mix and some pikes.
-> Keep cutting down trees as far as you can and let the grass expand till you are too close to a portal.
-> Make sure to expand if possible to unlock the Catapult and Pikes.
-> Make sure your outer walls are max. upgraded to Iron.
-> You can also upgrade towers till max. or the one before, but it can slow your progress down a couple days.
-> If you have 2 squires on each side with around 10 archers and the catapult, you should be save to attack your first portal.
-> It is possible to expand aggressively after a blood moon.
Day/Night 16 - 30+
-> Don't forget to put coins in the Hourglass Shrine.
-> Keep recruiting - mostly archers and some pikes if they lost their pike.
-> Keep cutting down trees as far as you can and let the grass expand till you are too close to a portal.
-> Make sure your outer walls are always max. upgraded to Iron before you attack a portal. It can be a smart move to find a spot where you can build two walls close to each other and a tower. It has happened that my first tower was destroyed. The second tower protected most of my soldiers when they retreated.
-> It is optional to buy some purchase some flame barrels before attacking a portal.
-> Don't attack another portal if your walls have not fully been restored. It is okay to wait an extra day for this.
When you have destroyed your fourth portal, you will unlock the Cracked The Skull achievement.