I was trying to get this achievement and realized there is a bug.
You need to defeat Sinister & Clones in X-Def 5 times with 7-League-Step as the finishing blow.
First time worked.
The second time I did this, the count did not increase.
The third and fourth time did not work either.
EDITED to include my findings:
I do believe I have solved the bug in this achievement. I believe you have to be wearing the default costume (black Marvel Now one)!
During a Sinister wave, I first killed Wolverine with 7LS while wearing the default costume. NOTE: I was wearing all of my artifacts/legendaries and used the skills that proc DOT effects. Got my Wolverine count up to 3/5.
Then I changed to the modern/classic costume and killed Colossus with 7LS. Colossus count did not go up and stayed at 2/5.
Next, i changed to the new Phoenix 5 costume and killed Cyclops with 7LS. Cyclops count did not go up and stayed at 2/5.
Finally, I went back to the default costume. Killed Sinister with 7LS. Sinister count went up to 3/5.
Wearing the default Marvel NOW costume, I ran it another 3 times and got the achievement with no problems. I would say it's safe to assume that the Marvel NOW costume must be equipped to get the achievement.
The idea came to me as I recalled completing the first attempt. I was in the middle of a prestige and couldn't use my classic costume, because I had a level 48 core on it. So I was using the default costume at the time. Wishful thinking lead to the assumption of the bug being due to the costume you are wearing.
I'm assuming that the majority of people like the Marvel NOW costume more than the classic one. Hence, it wasn't a problem for many. However, now that the P5 costume has released, this problem will likely occur more often