Happy Birthday Achievement in Vampire Survivors

This new enemy is only found in Lake of Foscari, and only in a very specific place in that stage. Basically, from the spawn, head east and then north until you get to the tunnel maze on that side. Head north into it, east to the second northward path and take it, then west, south, and loop around back out the northern exit. Head north a few squares from the exit and then east where you'll shortly find a rock wall with caterpillar-like enemies crawling out of it. These are the Sammies you need to kill. Refer to THIS LINK to see a map of the stage and see where exactly they are. That might be more helpful.

Once you get to that area, there is unfortunately no way to track how many you've killed while you're farming kills, and the achievement only unlocks after ending your run, so you have to guess. What I did was, once I got there, let my kill counter go up 30,000 to 40,000 kills, just to be safe. So, since I got there at around 10,000 kills, I hung out by the way until I reached 50,000 total kills, and then I quit and the achievement unlocked. If it doesn't for you, you can check in the Beastiary to see how many of those enemies you've killed before jumping back in to farm more.