I know there has been a few months since the last reply on this topic but I think I will weigh in. I started a new PSN (Target_Akawhired) to see how quickly I could earn shotguns playing only Solo and 4-player co-op. I started out with a solo match of Die Rise, reaching about Round 32 with 1400-ish kills, 1 down. I deliberately shot the Jumping Jacks each time they came to avoid obtaining Quick Revive or Who's Who as they both give you two downs for the price of one depending on the round you go down in. I don't think I have ever successfully revived myself with WW in solo.
Following that, I spent a few days playing Origins in 4-player co-op. I was playing with public match randoms, so I shopped for teammates until I found three players with headsets who sounded like they were very experienced at the map. This worked out well as I played multiple games over the next few days with 1500-2500 kills and between 0-2 downs. I did have like four or five bad solo games where I took a random early down (below Round 10), and some "average" co-op games where I took more downs than usual. My main emphasis during this early part was K/D. My secondary focus was making sure I revived teammates a lot. My revives was more than double my downs for the entire time I was working toward shotguns.
After about 8 days, I had a K/D of 392 (13,340K/34D). One of my friends was in a custom game on Die Rise, so I joined in. Due to joining in late and not spawning near the bank, I took 3 quick downs fighting with wall weapons, and a 4th a few minutes later. Every one else died at that point. I had 75 kills and 4 downs. Game ended, we get back to lobby, and I have shotguns.
I took my foot off the gas once I earned shotguns and my stats have come down somewhat. I went back to playing mostly solo, and have been attempting to complete the Origins easter egg in solo mode once with each character. I completed everything but the final step (releasing Maxis, ending game) as Richtofen but got greedy for rounds and kept playing. I died in the crazy place in Round 40. A half-day later, I ended the game as Dempsey and the length of the video was 115 minutes (proud of that one!). Since then, I have been attempting to complete what I call the "Drunk Russian Challenge"...which is obtaining all 9 perks AND completing the solo easter egg with Nikolai. To date, I've not been able to complete it but have gathered all of the perks once. It's difficult and tries your patience. What's this have to do with zombie rank? Well, needing to play as Nikolai, I have quit out of MANY solo Origins games when I don't spawn into Round 1 as him. Quitting de-ranked me back to the knife and I had to play for high rounds to get the shotties back, which I thankfully did. Now I just roll with whatever character I get in solo because the quits torpedo your rank more than anything I think.
It's either that quits are REALLY bad, or those quits are logging each time as a game that only went to Round 1 into my average round/match. Based on all of my observations over the past few weeks working with this ranking system, I would say the order of importance is as follows:
1. Kill/Down Ratio
2. Average Round Reached
3. Not Quitting
4. Reviving Teammates
5. Everything Else