Well, for Heavy Armor, don't you just need to boost 10 times in a match? That's not particularly hard.

As for Disguise, there's not much to say about it. Don't use it when you're obviously heading from where the enemy will be. Experienced players know how to see right off the bat that an enemy is disguised (ex: Helghast-impersonators don't have lit up eyes). Instead try to get to like a side of a level of where anyone could be going anywhere and disguise then. Time your shot wisely since your disguise will expire if you hit an enemy without killing them soon enough.

I've gotten the Platinum in this game - being in the top 1% of leaderboards in one week - so I can answer any questions you got, could be a little more specific, though?