Age of Mythology: Retold

Easy 5-6 minute method with the ingame editor

1. Start the game, click on the = (top right) and select editor.
2. After a loading screen you see a green and empty map.
3. Click on objects (top row in the menu bar) -> place objects
4. A new window opens. On the left side select player 2 and on the right side you can select the object type. Select villager and place 50+ close together.
5. Now switch over to player 1 and select the Fenris Wolf Broods. Place 5-10 next to the villagers.
6. Optional: Select water on the top row and 'draw' the water around the villagers and fenris units so no one can escape. This way all the killing is automatically.
7. The last step in the editor is saving. Click on file -> save (or click on button #3; the little disc icon) and type in a file name.
8. Now back out into the main menu of the game (file -> back to main).
9. Select skirmish and as game type (top left) -> custom scenario. Select the file you saved and start the game.
10. The Fenris units will attack and after 1-2 minutes the achievement will pop. If there are not enough villagers on the map just restart / reload the scenario.

This is an easy achievement.

Start a skirmish with the Norse using Thor
Settings:

• Standard Mode
• Game Speed Fast
• Infinite resources
• Reveal all
• 12 AI Players

Start:

1. Begin by building the temple and then upgrade to age 2, select any god.
2. Build all houses to allow for max troops/units.
3. Build the Dwarven Armory and upgrade to age 3 using the town centre, select any god.
4. Build the Hill Fort and then when upgrading to age 4 be sure to select Tyr, which will allow you to create Fenris Wolf Brood at the temple. Upgrade Fenris Wolf Brood at the temple.
5. Create an army of Fenris Wolf Brood and send them to each base to kill the villagers, you will likely have to manually select them to attack the villagers. Kill 100 villagers and this achievement will pop.
6. Build more temples to produce Fenris Wolf Broods at a faster rate if needed.

If you need more help with this you can follow my video guide below which will begin at this achievement:

Start a skirmish with as many AIs as your PC can handle on the lowest difficulty. Choose either Thor, Odin or Loki (Thor and Loki also get access to Bragi, which will help). Set ressources to infinite and if you are comfortable with it, increase the speed of the game.
At the start built a couple of temples and as many houses as you can. After the first temple is finished, age up.
Build a forge and age up, choosing Bragi, if possible.
Once you are there, built a fortress, research Thuriasz Rune in the temple and age up, choosing Tyr.
Once there, auto-queue the wolves. Once you reached max, go once around the map. Attack move them into an AIs settlement. They should have five-eight villagers. Once those are dead, leave the settlement and move on to the next AI. Depending on when you do it and how many AIs there are, you'll be done around the eight visited AI. Reinforce your wolves as needed.

The AI will not attack your base, so you don't need to worry.