TIPP: Achatinas can produce organic Polymer. You can just craft a fridge with the organic Polymer and destroy it. Every time you destroy a fridge you get normal Polymer back. That means you can transform organic Polymer to normal Polymer. I found it out yesterday by myself.
Don't worry about level when you're looking for them. Get any level, even a lvl 25 I got has 156 weight. Considering achatina paste weighs just 0.01. That's 16,500 paste in a level 25. But it will never reach that as you get like 60 paste per hour. Set them to wander and feed veggie cakes, once in a while. Veggies cakes last 24 hours+ in real time. So spoil timer is massive, make a cage and put cakes in a feeding trough as that has same spoil timer as the animals inventory. My suggestion tame about 10-20. That way you get anywhere from 600 paste to 1200 paste every hour, and suitably all snails below level 30 but since they're hard to find, swoop any and all snails. On an unrelated note if you want cementing paste and you're not high level enough for veggie cake production, go the south cave(hunter) with a wolf, saber with at least 1500hp. Run around the cave and you will get 10000 chitin. Hope this helps!