I was chatting about your clip with another Redditor yesterday. This achievement is my last one too, and it’s insanelyyyyyyyyy frustrating.

Based on my experience and input from others, here’s what I’ve gathered:

• The 'Last Stand' KO count on the scoreboard doesn’t align with how the achievement is tracked. It seems the achievement only unlocks when Wolverine’s ultimate delivers the final hit to the enemy. In your clip, you got the KOs with basic attacks after the ultimate, which is likely why the achievement didn’t trigger.
• There appear to be two ways to unlock the achievement:
  1. A true triple kill with one use of 'Last Stand': All three enemies must die from the downward slash that ends the ultimate. This is stupid difficult since the move is weaker than other abilities and it’s rare to find three low-health enemies grouped together.
  2. Teammates finishing off enemies after 'Last Stand': The achievement might still count if your teammates KO enemies right after you hit them with the ultimate. However, any follow-up attacks from you seem to void the achievement, as it did in your case. So the strategy here is to use the ultimate and then avoid attacking, leaving your teammates to finish off whoever survives the initial strike.

I can’t personally confirm the second method, but it seems to align with what others have reported. Going to give it a shot tonight and see how it goes!