Pengs treasure can be found in the hanger bay when you first arrive on the Ishimura. You won't be able to get it until chapter 11 though since you won't start off with Kinesis. Its in between the two walkways and it's a small golden statue.

Secret Achievements: Don't get cocky, kid: You have to make it through the ADS Cannon part in chapter 4 with over 50% shield remaining on any difficulty.

Slugger: Beat the slug boss with over 50% shield remaining. Good Luck.

Ok, I'm going to tell you guys some stuff that helps you stay alive-

-If a necromorph is laying down, don't turn your back unless you shoot one off the limbs because if you do, they'll get up and attack you from behind.
-Always watch the oxygen tank on your back. Here is a cheat to refill it continually- Pause game and press X,X,Y,Y,Y and you will have full health.
-Don't be fooled, the energy slicer may be the first weopon, but it is one of the best weopons you can use against the necromorphs!
-The best weapons against necromorphs are the energy slicer, the ripper, and the line gun, because they rip off the limbs the best.

I hope you enjoy these tips!

Make sure to get the one gun achievement (only plasma cutter) on your first playthrough because it is the easiest to use and pretty much the most effect for a large part of the game. And a quick deterrant. Dont even buy another weapon on the first playthrough. If you shoot ANYTHING from any gun besides the plasma cutter, it will deactivate the achievement. I'd know. I just beat it today killing only with the plasma cutter and still didnt get it. As for the rest of the "30 kill with this weapon" achievements, carry around stasis packs and use stasis before trying to get the kill. Most guns besides the plasma cutter take a while to get the enemies down. The contact beam charges, the line cutter is a slow projectile, the flamethrower takes a bit of time, and the ripper is a bit confusing to control at first. As for the force gun and pulse rifle, stasis still helps because they aren't the most efficient killers when you're attempting dismemberment.

Hope this helps out in the long run. =)