I think I've found the answer to your question in a response from a developer to a similar topic. Pete DonnellyPosted 03 June 2015 - 01:50 PM
You rank up for winning - Killing all 5 nests without dying 10 times.
If you lose, it's possible to rank down (as you've seen). You do rank up faster than you rank down.
You also need max Survivor level in the skill tree before you can get the gold weapons. Then it is possible to acquire 3 gold weapons as you rise through the Human PVP ranks. (4th, 8th, and 12th rank I think...)