Start a match in Castle Chaos

This one is straight forward... just play a match.

100 gnomes killed.

To kill a gnome you have to shoot him with a cannon (building a wall around him doesn't count). The gnomes can be hard to hit and they'll laugh at you when you miss. I recommend setting up a custom match with the max gnome density (20), max amount of cannons per castle (8) and the max attack time (99). Put the cloud density to 0, castles to 2 and the rounds to 10. Make all 4 players human controlled (and just ignore the other 3). Then start the match and blast all the gnomes you can in each players section. Have fun!

Capture all castles in your kingdom

Set up a custom match with max castles at 1 and put the building time up to 99. Then capture both of your castles.

Defeat one or more AI players on difficulty setting "Easy".

This shouldn't be very hard but it'll give you a good intro to the game. See the "Defeat Hard AI" section if you need help.

Defeat one or more AI players on difficulty setting "Medium".

Once you understand how the game works, this shouldn't be too hard. See the "Defeat Hard AI" section if you need help.

Defeat one or more AI players on difficulty setting "Hard".

*If you go for the achievement "Unfair Challenge", you will get this one at the same time.

The easiest way to beat a hard AI is through score. Set up a match with build wall time at 60 and attack time at 99. Set bushes, cloud density and gnome density to 0. Set the number of castles to 2 and the cannons per castle to 8. Pick 3 or 4 rounds. Now start the match and focus on destroying all of the hard AI's cannons. It takes 20 hits to destroy a cannon and your score will go up by 200. You should be able to wreck most, if not all of his 8 cannons by the end of the first round. If you have time to spare, shoot at his castle after, as destroying a castle is worth 500. Now repair walls and try to get a second castle (leaving room for your 8 cannons). Start a wall around the third castle but don't finish it; it'll be a backup for later. Now attack the hard AI's new cannons... and repeat.

Defeat three AI players on the "Hard" setting.

*If you go for the achievement "Unfair Challenge", you will get this one at the same time.

If you want to get this one on it's own, just remember that the AI don't need to be on a team, so they'll be going after each other as well as you. Refer to the "Defeat Hard AI" section for game setup and strategy.

Succeed in repairing your castle walls by controlling a gnome.

If you get knocked out of a match that still has 2 or more opponents fighting, you will take control of a gnome who can build walls. Set up a match with a gnome density of 1 and 3 easy AI (no teams). After the attack phase, repair all gaps in your walls but one. Then, after the round ends, you'll take control of the only gnome in your area and he can build walls when you push the attack button. Now just repair the last gap.
If the AI don't attack you then gang up on one and destroy all his walls. That should turn someone's attention to you.

Destroy all castles of an opponent.

*This achievement is glitched. And the dev(s) seem to have abandoned the game. If anyone has been able to get it, please leave a comment with how you did it.

Defeat a team of three hard AI opponents.

Refer to the "Defeat Hard AI" section for setup and strategy. You may want to increase the rounds, but it's not necessary. The hard AI will usually blast away all of your walls, so it isn't too hard to rebuild since you have open space.
This is what my game looked like after 2 rounds:
As long as you can get a castle after each round, you'll win with lots of score to spare.

Three human players team up against a hard AI opponent.

Refer to the "Defeat Hard AI" for game setup and strategy. Add in 3 human players on a team, and one hard AI. Don't worry about the other 2 and hope that the AI goes for them instead. Only one human needs to survive to win.

Kill a gnome just before he places a bush.

*You will likely get this one while you are working on "Gnome Killer".

When a gnome is about to plant, he will hold the bush over his head for a second or two before planting it. Just shoot him after he has a bush but before he puts it down.

If you found this guide helpful, please give it a like. If anything is unclear, or you see any mistakes, please leave me a comment and I'll try to work on it. Thanks for reading!