I just wanted to confirm that you can hop off the Panzerhund and you MAY kill enemies while off of the hund. The achievement will pop once you've reached the water (and are swimming in it) in the sewers.
A couple tips:
Utilize manual saves every 2-3 enemies. You will thank me later.
There is a safe space with max health and 100 armor to the right of the second Nazi vehicle thing. As far as I know, it is only accessible if you have the constrictor harness contraption.
There is armor and stuff hidden around the map in crates just like a normal level. You will need it if you can't stealth it (which is next to impossible).
Try to get to the first checkpoint ASAP since the game deloads all the previous enemies whether they were dead or alive, so you are free to go back and get pickups and collectibles without interruption.
Finally, you can stealth the sewer section very easily without ever firing a shot. Since there is a Zerstörer down there I would DEFINITELY recommend doing this.