To become friend with the Jötnar Tribe, you must have a market to trade them food for money. It take a while for them to become friend with you, granting you one giant as warrior unit, and the achievement. Like others achievements, it's easier to do it in a solo VS 0 map.
Easy "The Mystic" Achievement
To take control of the Yggdrasil zone, you need to kill the Valkyrie in it, and pay 2000 food to take over the land, granting you a victory and the achievement.
Easy "The Smith" Achievement
You need to have a magma tile on the map (random event tile with every new game) then you can build the forge (Advanced building) and then you have to craft the three pieces of the sword in a row. It costs iron (fifteen for each piece for a total of ...fourtyfive yeah you good). Then you start crafting the pieces, be ready to defend the zone cause enemy will try to conquer it, and no matter how many pieces you already craft, if you lost area control, you have to re-craft all three pieces again.
Extreme Mode Strategy
Be a save-scrub champion. Whenever you're on a tricky mission, save every other month. Since Northgard implements semi-random events and world hostility, going back to an earlier save may change where / when Wolves and Draugr invade, which bad event (Rats, Blizzard, Earthquake) comes up, etc. In some levels, you need to do as well as you possibly can throughout the mission in order to beat it. The idea is to save as soon as you've done a perfect iteration of gameplay; that way, if something happens in the next couple months, you have a 'near-perfect" save to go back to.
Time is of the essence. You are allowed to relax in Skirmish, but in Extreme, you should always be micromanaging your military or villagers to be as efficient as possible. Pay attention to notifications, especially idle villagers. Turn scouts and warriors into villagers when you can; don't retain an army unless necessary or you can't afford to disband them.
Maximize efficiency. Make sure you leave room for silos on food tiles, place Markets and Trading posts on the same tile, and leave room for a Mine when Stone / Iron is on the tile. Pay attention to each clan's Lore when planning out what to colonize.
You should almost never need to develop land (paying 100 Krowns) unless you are Goat and can do it for 20. Instead, push food production and expand to a new tile.
Resource Management
Expand until you can have a steady supply of everything you need. On Extreme, you rarely need to take more than 5-6 territories besides your home province.
Make sure you store nothing but the bare minimum of resources! Food is good for colonization and not starving; nothing else. Wood should be used immediately; if you can't use that 150 wood immediately because you don't have building space, you need more food to colonize another tile. If you have over 300 food or 200 wood and Winter isn't coming, you're not spending your resources in a timely fashion. Pretend you're a project manager, and your boss is going to take your budget away if you don't spend it.
Krowns are nice, but unless you are playing Raven, you only need enough for upgrading buildings and hiring a few military units. Try to keep about 100-150 in reserve with at least +2 production and only push Krowns if you need them for Upgrades or Military.
Mine Stone and Iron as early as possible on most maps. Upgrading buildings is vital.
Trading Posts are the Great Equalizer. If you have a map rich in forests, or food, you can sell off the excess from your efficient, upgraded buildings instead of posting villagers as Merchants. Depending on the map, you can get 2-6 trade routes going at once, though you rarely need more than 2.
Don't buy resources from the Market! It's almost a guaranteed sign you haven't expanded properly or in the right order. The campaign ensures that you are near all vital resources, so don't buy from the Market unless absolutely necessary!
Upgrade buildings and tools
Upgrade buildings relative to what you need as quickly as possible. If you're running low on Food, upgrade a Food building. There are three essential tools to upgrade with Iron every level: Woodcutters, your biggest group of Food Gatherers, and Axe Throwers.
Various Steam Achievements
Complete the indicated task to unlock the achievement.
