Age of Mythology: Retold

I just now did it in 12 minutes. Start a skirmish with one AI on lowest difficulty. Set ressources to infinite. Go with Loki or Odin so you get access to Heimdall.
At the start auto-queue Berserks and build temples, go to the next age choosing Heimdall. While that is happening, build houses.
Once you reach the second age, auto-queue Einheri. You can manually trigger their ability. I just waited for a moment to have 20 of them, then manually trigger their skill by selecting them all and spamming left-click on their skill. Done.

This is an easy achievement.

Start a skirmish with the Norse using Odin
Settings:

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

Start:

1. For this one you can gain another achievement during this, see my video guide below for more info.
2. Begin by building the temple and then upgrade to age 2 and be sure to choose Heimdall which will allow you to create Einheri at the temple.
3. Build a few houses to allow for more troops, no need to max out.
4. At the temple create Einheri and once you have a few, click on them and activate their Horn Blast ability. Do this 20 times for this achievement.

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