Kill 25 enemies while being dragged from the restaurant, without dying or restarting checkpoint
This should be the very first achievement you'll unlock, as it's the very first part of the game. You'll follow your friend Vinnie into an Italian restaurant where you'll start speaking with two ladies. The Mob will then blast in and damage your leg. Vinnie will try to drag you somewhere safe and you'll have to cover him by shooting the enemies. For the achievement, you'll want to kill 25 of them without dying or restarting to the last checkpoint. There are more than 25 enemies but you won't be able to kill them all. What I suggest is that you kill as many as possible while you have the one handed pistol. When you'll have dual pistols, you won't be as accurate and you'll miss plenty of targets. When you play on easy, it takes only one bullet to kill an enemy. If you fail, simply restart the level in the pause menu.
As you have infinite ammo don't waste your time aiming.
I'll describe checkpoints for you to see if you're on the right way:
From the beginning to the Candlestick falling on the Italian guy: 9 enemies.
From the Candlestick falling on the Italian guy to the white van crashing into the restaurant: 6 enemies.
From the white van crashing into the restaurant to the end: 10+ enemies.
#13 is right. I just got this achievement, and I forgot about [supposedly] not being able to restart the checkpoint [which put me right at the part where you get handed the first gun]. It may depend on how far along you are when you reload the checkpoint. If you reload from the last checkpoint as you are being dragged into the kitchen (just before the "new" checkpoint), it probably WILL fail!
It was JUST after the prompt about the "iron sights" that I realized I might not have missed as many, had I looked down the barrel for more accuracy! If you have missed more than 5 or 6 guys by the time you're handed a second gun... you might want to restart the last checkpoint. I did that, and by the time I reached the kitchen (still shooting as I was dragged-in), I got the achievement!!! :