Marvel's Midnight Suns

If you want to get this over with at the earliest possible, and not wait until THREAT rooms open up, the first encounter on the final Deadpool mission works really well for it. I'd imagine his first two missions might work well for it too, but I didn't get it until the final one.

Just concentrate on eliminating, or taunting with other characters, anyone targeting Deadpool and use a lot of cards with quick kills to keep as many of your card plays per turn as possible. It takes a tiny bit of luck to get the right draws from your deck to start, but it is doable. As long as you keep Deadpool from getting hit it's pretty easy to set up. I believe any kind of armor types of cards prevent losing stacks upon hit too if you have any.

Make sure to keep some of the elites alive so that you keep getting enemy reinforcements. Some enemies will also resurrect randomly each turn. That also helps get the kills needed because they're usually ones that only take one hit to kill.

The first kill will net him 1 En Fuego stack, the second and third kills will net the 2nd En Fuego stack, 2 more for the 3rd, 3 for the 4th, and 4 for the 5th and final stack.

Otherwise some of his upgraded cards do make it so that your teams kills will count towards his En Fuego stacks so that will make it easier once you get them. But those dont unlock until later I believe. Environment kills do not count for stacks though.

Hope this helps somewhat anyways.