Call of Duty: Black Ops 3 Zombies (DLC #3 Descent) - Zombiepult Trophy / Achievevment Guide - In Gorod Krovi, launch 10 zombies into the air at the same time. [Bronze]

Step 1: Get GKZ-45 Mk3 / Monkey Bombs / Dragon Strike (only 1 of these 3 is needed)
Step 2: Activate power and run in big circles around the Flinger trap (trap near the power switch)
Step 3: Use GKZ-45 Mk3 / Monkey Bombs / Dragon Strike on the Flinger Trap and quickly activate it! The GKZ will free the zombies. The Monkey Bombs / Dragon Strike will lure them to it. Without these weapons it would be pure luck to get 10 on it at the same time.

You can also use the Bridge Trap for this (the one for 500 points between infirmary and armory). I found it much easier with the Flinger trap as it is bigger and has more space to round up zombies. It is easiest with the Monkey Bombs, though they are very rare to get (I got them once in ~100 mystery box uses).
There are also some gobblegums that freeze or slow down zombies

---