6502 ASM is your friend.
I authored a series of (PG-13 at best) tutorials on this subject a few years back. If you're interested, I can send them to you.
Honestly, it's not as formidable as it may at-first seem. Someone with average intelligence, who is honestly motivated and actively goes at it, will attain some manner of proficiency within 1-3 months. Various degrees of mastery, 2-6 months.
If you've ever played Donner Party, I was the sole ASM wizard on that project. And when I started, I had barely a year of ASM experience under my belt.