I don't think there are actually 20 Zone Dogs in the main campaign. I thought I killed every enemy, but I was 3 short. There are two good ways to do this, though, even given that:
1) If you're still in the early to mid main campaign, there are a couple sections where you run across three Zone Dogs together. Save the game, kill them, reload the game, kill them again, etc. etc. until you have the trophy.
2) The DLC has Pod Wolf enemies. These appear to count as Zone Dogs, since after I killed three of them the trophy popped for me.
Alternatively, you can probably start the game over and find/kill some dogs, but I'm not sure how far into the game it is before you run across some.