Age of Empires IV: Anniversary Edition

To get this, you just need to use one of your Kahn's arrow powers with 100 of your units within range. It does not matter which of the three powers you use.

Fortunately, the game helps you out. The range of the Kahn's power is pretty far and it shows the reach of the power with a circle centered around your Kahn. You can see how many units are within the Kahn's range by selecting them, then do a quick count to make sure you hit triple digits.

I completed this on a single player skirmish against one easy AI, although it took a couple moments before the achievement popped. If you're unsure, the arrow powers recharge after only a minute. Just sit tight and try again.

Edit: To clarify, the arrow must actually affect the units within its radius. So for example, using the arrow speed upgrade will not affect knights, but it will boost Mangudai.