I think this picture will explain the problem.
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It's hard to see, but the achievement clearly say "Kill 150 people with remote mines", but in progression bar, it's 1/50...
Also, Boris isn't invincible at all : I was supposed to got this achievement yesterday but nothing hapenned... I think there's a problem over here.
As for the progress bar, it clearly is a bug that will probably be fixed on the next patch.
Please explain what you mean by "Boris isn't invincible at all". Does that mean you can't get the achievement because you don't meet the requirements? If it is this, then all you can do is try harder. If you meet the requirements and still don't get it, then I guess we have another bug at hands... I think it might have to do with a problem related to the character change. Sometimes when you change your character to Boris, it registers as being Oddjob. I guess this was already reported here in the forums.
PS: I noticed that you already have a readily correct French translation. It is funny because the achievement "Dutch Rudder" is translated into French the same way it is translated to Portuguese!