1 point of protection subtracts 2 crit percent points. That 20 prot means you need a monster to have a 41% crit rate to have a 1% crit chance on you. For the capped 30% effective rate, you'll want to find a monster with a base 70% crit rate (or like said above, do things to nullify your protection, including fighting monsters using skills that ignore protection when calculating critical like Windmill).
But why are you trying to master Heavy Armor Mastery anyway? Shield Mastery's mastery title is actually better (for some reason), and mastering a skill gives no more talent EXP.
For beginner survivability, heavy armor mastery isn't a bad choice compared to the AP cost.
If you want protection off, just go master smash, it's really easy.