Assassin's Creed Valhalla

This achievement is cumulative across all runs of the DLC.

The Kick of Tyr, like all abilities in this mode, is obtained randomly during runs. You might start a run with it, might find it from an encounter during a run, or might not see it at all during a run. Once you do find it (you assuredly will during one of your multiple runs required for all achievements), you should make it a point to go for this achievement immediately. However, enemies in the third region, Nidheim, are extremely strong and tough to progress this achievement on, and the fourth section has barely any opportunities for this achievement. So, if you find this ability late in a playthrough, consider spending the 75 Memories with Nar in Odin's Camp to take this ability with you on your next run. The enemies in Kaldstad and Dokkerland are far weaker, and they are almost always on platforms that they can be kicked off of.

What is strictly required for this achievement is that the enemy is kicked off an edge and that they die from the fall damage. If the kick kills the enemy first, or they hit a wall and die before their body plummets, it won't count. Thankfully, pretty much every enemy in the first two areas will die pretty easily from fall damage. It then just becomes a matter of managing your adrenaline bar(s) so you can use the ability. An assassination kill will fill an adrenaline bar, so use that to your advantage to get one or two kills towards this achievement in each encounter, and you should easily be able to unlock it in a single run. If not, you should earn plenty of Memories in a run to pay for the Favor from Nar to take Kick of Tyr with you again, so you don't have to worry about finding it again.