If you think about it, against armored units stalkers are the weakest.
marauders > roaches + stalkers
20dmg > 16dmg > 12 or 14dmg (forgot)
Usually the reason stalkers can > roaches is bcoz of > micro and blink or some help with pure numbers since they outrange roaches by 1
This is pure stalkers, with no other units mixed.
So if you have roaches, you just need to a move- then walk up closer, a move, reapeat.
Speed roaches will wipe out normal stalkers and blink stalkers, but if the micro is to great then its time for hydras.