CastleStorm

Map: Donkey Island – Battle Isle
Your Nation: Kingdom (Recommended)
Castle Position: Left (Recommended)
Enemy Nation: Norse

Difficulty: Hard
Castle: Skyhaven (or Custom)

This was my personal set-up, and my recommended upgrades, although higher levels of everything will make it easier:
Projectiles: Javelin, Apple Grenade (LV5), Tri-Stone (LV10), Battering Sheep (LV5), Potion of Conversion
Spells: Sir Gareth (LV5), Heavenly Swords (LV5) – Others are up to your choice
Troops: Griffin (LV5), Stone Golem (LV3) – Others won’t be used
Rooms: Armory (LV5), Pantry (LV8), Training Room (LV3), Kingdom Flag (LV10) – Others are your choice

Tactic:

I recommend destroying the castle, as capturing the flag is much harder. As soon as an enemy spawns, use your javelin to kill any enemies you can. This will help build food as well as give time for your other projectile ammo to charge.

From here, what I did was spawn Stone Golems and Griffins when I got the chance, and protected my personal base as well as I could when there were stray enemies. Keep spawning these two troops whenever possible. Also, make sure to use potion of conversion when you get the chance on big enemies.

Now that you’ve protected your base, and built up the ability to shoot your ballista, it’s time to attack the enemy base. ALWAYS shoot your javelin first to aim before you use any of the better projectiles, so that you don’t waste anything. Once you get the correct aim, you’ll want to put the battering sheep into the bottom or middle of the castle, and use the grenade apple and tri-stone to help take it down.

Here’s a huge helper. Remember that getting a 5 kill streak will allow you a brief period of infinite projectiles. The easiest way to get a 5 kill streak is to let some enemies bunch up near each other on the battlefield and then use your tri-stone to aim low, exploding early and letting the stones travel along the entire battlefield clearing people up. When you get infinite ammo, quickly making sure to aim your tri-stone at the enemy castle and shoot as much as possible. Remember, it’s not a bad idea to also aim some stones on the battlefield to clear out the enemy troops.

Taking down the castle will require some work, so make sure you are using what’s available to you, by spawning the griffin and stone golem when you can, and also using the heavenly sword if things get out of hand. Lastly, it takes a while, but use Guardian Spirit if and when things get really crazy.
With this method, I was able to pretty easily beat the Hard AI in Skirmish. I took down their entire castle, while I was still at 8/14 rooms. If you are struggling, remember to just buy some extra upgrades, and try again.