Railway Artillery or Stuka Bomb is sufficient to kill it, along with almost every unit that the Ostheer provides.
It takes a century to turn the damn thing and a decade before the shot lands on the ground since you ordered it to fire. You need to be very lucky to get it to veterancy 1 and after that it gets a little easier, it's still as slow though. You need to predict where your enemies are so that you can shot with it as soon as you see their units.
After +20 games trying very hard to find use of this thing I decided that I would always go with the 152mm Howitzer since you are more likely to kill something with it, for the same cost.
The only really good use I've found for this thing is to make precision bomb strikes on camp fires during blizzards. Just make sure that your B-4 are aiming at a camp fire before the storm comes because if you have to move it in order to shoot the blizzard will be over and their units are not camping around fire pits anymore.
