I have 7 achievements left to get and this one looks like it will be the most difficult. I am doing Legendary co-op right now and it is proving to be rather easy (Arcadia City and Scarab were somewhat annoying but managable).

So, any tips for this? I know I have to do all missions UNDER par time as opposed to being in the par time.

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I 4 shot with both my BR and my MAC.

I got that achievement by completing the campaign on Legendary normally. Didn't even try to hurry. Don't know what you mean in the last section of your post as you don't need to do them UNDER par time, as long as you gain the little clock icon after completing a mission on Heroic+, you'll get Mr. Punctual. That's the achievement you're after right?

 

Unless you're on to something else, then I've totally misunderstood your post. :P

Well, each mission has a par time. Let's say, for example, that one mission needs to be completed in 10-15 minutes. Does that mean I have to complete the mission at 9:59 or under?

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I 4 shot with both my BR and my MAC.

If I remember correctly. You have to complete the missions under par time.

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I got the achievement just yesterday, did the Rhino mission and completed it just below 30 minutes, think I did it at around 25 minutes. I believe under par time would have been under 20 minutes (not sure).

 Anyway, just complete it on Heroic on the times that are displayed between XX:XX-YY:YY

 So if it says par time: 20:00-30:00, then you'll get it if you complete it in 22 min, 23 min, 29 min.....