The requirement for this dark ops is to reach round 25 on Tottenreich without getting freeze.
I saw the requirements for this on twitter. But in this game i was just playing to leveled up the peacekeeper (its on the wall) but i poped up a flavor hex and get hidden power so i give it a try.
Anyways, my strat was that on round 6 i pass the round on the church (zone of mule kick). Later, i pick up the chains on the ship for the ww quest and opened the zone of the constelations step. Then every 5 round i flipped the round on that zone and is super easy to no get freeze in this zone because inside are only 3 spawns and the 80% of the zombies came from the stairs.
Keep in mind that the freeze zombies have an abillity to create an area that freeze, so dont get too close to them and for me until round 23 only spawned one of this zombies per round. They make a sound similar to the special round on shattered veil so keep atention to that.
For normal rounds i camped in the zone of mule kick were the zipline is because no zombies spawn behind you. In the last three round i just spamed kazimirs when i hear a freeze zombies.
Last thing is that on round 24 i get like 3 of this zombies so pay special attention on this round.
My main language is not english as you can see so i hope almost everything is understandable.
Hope this helps and any question i would try to answer.