To get the meta game achievements you should use some of the skulls although they make the game more challenging they can give multipliers of 1.5x and even 3x.
Another tip is to do the level on heroic with 3 skulls activated this normally gets around 17,000 to 22,000 points which is more than enough to get the achievement as you only need 15,000 points (unless of course you are doing the 2 missions that require 50,000 points then you usually get in the range of 52,000 to 60,000)
You can do it with your friend just make sure when you turn campaign scoring on it is set to "Team" not "Free for all" otherwise its like competing against one another which is pointless. Ending up with you both having no way near enough points to unlock an achievement.
The person above me is wrong it is actually much faster doing it with someone just make sure campaign scoring says "Team".
Hope this helped
P.S. To solve your problem about not getting the achievement when you had the required points. You may have been playing 1 of the 2 levels which are exceptions to the 15,000 point requirement these levels need 50,000 points to unlock the achievement. The levels are "The Ark" and "The Covenant" and I believe you need 50,000 points for these levels because they are massive and you get to use alot of vehicles.
