It's almost never 'assembly expertise' - it's architecture expertise - knowing a specific instruction you want to use in a specific way to get some kind of fusion or fill some functional unit using some knowledge that the compiler doesn't have.

The actual mechanics of how to write a program in assembly are trivial if you already know a language like C.

> trivial if you already know a language like C

The few times I've been stuck writing code for a platform without access to a compiler, I just wrote it in pseudo-C then hand-compiled it. Once I got comfortable I could "compile" on full mental autopilot, checked out and watching TV at the same time. But it's horribly dull work, like doing arithmetic by hand.