This isn't exactly a guide but more of a tip for any levels you may be struggling on. I finished the campaign getting golds on most of the levels without much trouble but there were a few levels that I couldn't get gold or even silver on.
After earning a good amount of xp from assault ops and challenges, I started using score booster and that helped me get golds on some levels but i still couldn't beat some even with the booster.
The key I learned is to actually use damage booster instead, it's 200xp cheaper and allows you to kill covenant much faster, thus leading to higher scoring medals and more of them than you would get by using score booster. Couple this with using overshield, which makes you nearly invincible when used right and you score well over 100k points more than what is required for gold. Hope this helps!