While there's a few methods of getting this, easiest is likely going to be the 'Crestfaller' gun. It has very low damage, but very high freeze capability. I've seen a few now, and it's what I ended up using.
But you've obviously got to get one of these first, so realistically any gun you get with freeze capability, you'll want to use. My personal caveat, however: the first Freeze gun I got was a Glacier, and I just couldn't get the achievement to pop despite rolling through several enemies frozen by it (which was very difficult, I might add; much easier with the Crestfaller). But first try with the Crestfaller and I got it.
It's not very hard, and you'll get it eventually, so take my advice: don't worry too much about this one, and don't be discouraged if you can't get it to pop with a certain freeze gun. There's always another somewhere in the Gungeon.