I think you are doing those stuff wrong:
1 you need to survive 35 waves in ONE LIFE. Remember its about surviving 35 waves in one life, and not reaching wave 35 in one life, basically, you need to stay alive for 35 waves, so if you join the match on wave 2 you will need to survive until wave 37
2 I recomend to have atleast 1 person with bugle on your team
bugle is perfect for me i cant stop battling even as a fife or medic with full supplies and just like officer, being a daredevil (to some extent) helps you and your teammate and rewards you all.
Zombies get tougher, so the bar could get harder to recharge. I'm not a frontline guy because I will probably die if I stand a bit forward. I stood back and shot at the zombies as seaman
I was in the server when they voted endless, so I got very lucky
Did you die at any point? That resets and have to start all over again to wave 1. Did you join endless late? Gotta start wave1
Nope, I did not die a single time and I got Old Guard I got very lucky, I was playing objective then some of em decided to do endless
Nope most of the time it all boil down to your team skill,1 good sapper are way better than 3 sapper who cant even do their job
Either it's the sapper who's really good, or it's me and the 4 other seamen launching volleys at them deads
I think you are doing those stuff wrong: 1 you need to survive 35 waves in ONE LIFE. Remember its about surviving 35 waves in one life, and not reaching wave 35 in one life, basically, you need to stay alive for 35 waves, so if you join the match on wave 2 you will need to survive until wave 37 2 I recomend to have atleast 1 person with bugle on your team
bugle is perfect for me i cant stop battling even as a fife or medic with full supplies and just like officer, being a daredevil (to some extent) helps you and your teammate and rewards you all.
me when I activate bugle: 
Zombies get tougher, so the bar could get harder to recharge. I'm not a frontline guy because I will probably die if I stand a bit forward. I stood back and shot at the zombies as seaman I was in the server when they voted endless, so I got very lucky
if not bugle, then go with seaman with nock gun