You don't need to do this to get the N7 challenge ...ok maybe if you JUST want that challenge for banner or something, but playing naturally and just promoting when you hit 20 will make you get this challenge before getting some of the other challenges like spectre mastery and all of the challenges under the aliens tab.
Just the challenge achievement, yes. I just don't like playing Gold with characters I'm not good with because I look stupid for how long I've been playing. I pretty much only play as a Kroguard unless I have a lot of IV equipment for my Huntress...and I'm by no means anywhere close to an expert with her, either.
And we all know how long Bronze promotions take...
M4A78T-E | AMD Phenom II X4 955 Processor 3.21 GHz | 16 GB DDR3 (4 x 4GB) RAM | GeForce GTX 560 Ti 1024 MB