This is mainly down to luck. There are 6 random weapon boxes located around the map, starting by the power supply.
You'll need the ray gun, the wunderwaffe, and the monkey bombs in your inventory simultaneously. The monkey bombs take the place of special grenade if anybody was wondering how you could have all 3.
For some people this will cost 50000, for some 5000. The first time i really tried for this I was at the point of 2/3 weapons and 30000 to spend on random boxes (with a crawler so the round didn't finish). I couldnt. get. the wunderwaffe.
The next match I played my first random box was the wunderwaffe, and on round 12 I got the monkey bomb and raygun in a row. Just goes to show, it's all about luck.
Just keep on using the boxes until you have all 3!
Note: I'm unsure as to whether there can be more than 1 wunderwaffe in the game at one time. I'm not certain but it might be best for you to give up trying for this achievement if somebody else gets the gun before you.
HOWEVER, if somebody dies and loses the wunderwaffe, you are still able to get it from the box, as I had it, died, then re-got it again in the same match.
Hope this helps