You see...my main issue isn't just surviing the Napads from kill 50-140, it's keeping them alive. THEY KEEP KILLING EACH OTHER!!! How many should I try to hold for a decent score? And what should I do with Gnezdo? Hold it or kill it ASAP?
From some of the Duo videos I've seen on YouTube. It's better to eliminate Gnezdo right away and hold the 8 Napads. On some it looks like partners split them in half so babysit 4 on the lower level and 4 on the top. Not sure what is best for Solo mode though.
Good news for you is that Default Sherry does real well on J'avo maps so holding Napads on Steel Beast won't be an issue compared to Urban Chaos where her counters are so weak during the chaotic enemy waves. When in doubt on Steel Beast, hop through ledges to get around to abuse iframes so you don't get pummeled by Napad randomness. I think the only annoying part is trying to shoot the flying bat J'avos since Sherry has no sniper but everything else is smooth sailing for her.
You could probably kill 1 napad if you really need to. If you run to the second level at around 55 combo, two balloons spawn from the hut and a few spawn at the bottom level and then the hooded J'avo spawn at the bottom. The napads shouldn't be too problematic at this point if you know and pursue the spawn points, there's plenty of environmental barriers for them to run into. They should always be lagging behind because enemies spawn at opposite ends of the bottom level and at the top of the stairs where the machine gun J'avo camps.
Can't say exactly in the very beginning, but then the first area is on the 2nd floor by the red barrel in the corner, near the stairs that go up and down, (opposite of the timer furthest away on 2nd floor).
Once the flying guys and arm guys come, you have to transition to the top floor. They'll all be there. Once the gorillas come, head down to the bottom floor and you'll be there for a while. After the cheap shark things with the orange ball in the middle, those blowing up balloon guys will come. This is usually the hardest transition as you need to make your way to the top floor for the final wave.