Skill & knowledge (this wiki, YT tutorials) is a massive power spike (^_^) Therefore you will see significan't boost every time you learn something meaningfull.
To say it simply there is no reason an MR20+ couldn't get his ass kicked on earth's single digit (level) missions - so no, no mastery rank could save person without skill or/and knowledge - but now you know at least that :p
Ps. If something is difficult there is usually a way to make it easy (that is not simply blind grind). Like for you I would throw a guess you could probably use a rhino, a shotgun, an sentinel and either 3 left or 3 right of 6 available sindicates conscripted to.