Not a game developer (although I'd love to be one) but If I were to make a survival game I would make the end game enemies more nightmarish & hard to read and more difficult to take on individually, maybe they have vigorously twisting & swirling tentacles that requires you to shoot/cut them off before going for that close up
shotgun-to-the-face or
magnum-to-the-weakspot etc.
Imagine a zombie game where all the end game zombies look like this, with each zombies having different style of mutations that requires some different approach:
Artist Ben Wanat aka the Dead Space god
This essentially makes end game zombies mini-bosses, and each one has a trick up their sleeves (venom spitting, grab attack, throwing up parasites, crawing on walls & ceilings, splits in two halves when critically injured etc.)
You fight about 10-15 of these unpredictable beings in the final chapters, all of them somewhat different and no more than 3 at the same time, and then the game ends with a cool final boss, gg.