Age of Mythology: Retold

Getting 4000 HP healed will happen rather quickly. I got it during the fourth Scenario of Golden Gift campaign. You automatically start with a Valkyrie and half way through the scenario, she alone had already healed that amount.

If you want to speedily get this achievement, start a skirmish with Nords as anyone but Loki. Set AI to the lowest difficulty, ressources as infinite. If you are comfortable with it, increase the games speed. Choose Freyja as your first Godess. You can upgrade them to increase their healing, if you like.
Then just fight a lot, keeping your Valkyries on passive, so they don't fight but focus on healing and you should get it in 20-40 minutes, depending on how practiced with RTSs you are.

This is an easy achievement.

Start a skirmish with the Norse using Thor (Do not choose Loki)
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 and be sure to choose Frejya which will allow you to create Valkyrie at the temple.
2. Build all houses to allow for max troops/units and make any upgrades at the temple for this troop type.
3. Build Valkyrie via the temple and then send an army of them to the enemy base, from here let them get attacked and they will self heal. Do this until the achievement pops.
4. Build more temples to produce Valkyries 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: