it's kinda hard to stealth the game, you're too easily seen, when you drop down on them you need to wait for a pretty good distance which is kinda far so the target gets to the point where it turns around and sees you. The bell is kinda too close to be honest, they just see a bit and reinforcements are immediately called, you cant chase them, you can still save yourself with the tendril yank but sometimes it missed. Especially when you're fighting two. They call reinforcements and while you kill those reinforcements another guy goes to a bell and calls for more, its kinda hard to kill them all at the same time.
Also it's kinda tricky to go grab those ledges haha. You really need to distance the impale so that the enemy will be near the edge (the green part) of the impale so that the enemy will get hit. Does assimilation regen health?
Maybe I'm just terrible at the game which it most likely is but here's my "opinion" anyway
Here's someone who did it right. I'm actually surprised no one went to ring the bell