Shadow Complex

Spread throughout the game, there are a total of 125 items. This number includes certain items that you get during the campaign (flashlight, guns, etc.) and other optional gear (Fusion Helmet, Hook, etc.). However, the vast majority of these items are pickups that help upgrade your maximum ammo capacity, health, and armor. You can view a more detailed spread of these pickups (including overall %) by pressing select and then LB, while in the game. This will be helpful when trying to figure out what you are missing.

Those of you not familiar with the Metroidvania genre might find its jumbled exploration a bit confusing, but here is a little tip to help clear things up. You need to keep in mind, that you will not be able to access every area at first. You will come across dozens of areas that you will be unable to reach, until you get the necessary equipment required to open them. This might be quite awhile, as there are several key pieces of equipment that you won't receive until much later in the game (Missiles). If you can't figure out how to get a certain item, don't spend forever trying to get it. Chances are, you are missing the specific piece of equipment required to reach it.

You might be a bit overwhelmed at first, when you start to realize how big the map is, and how elaborate the corridors might be. However, this game is a little bit more forgiving, when it comes to finding every little item spread throughout it. Once your character reaches level 20, you will unlock the full map along with the item locations. The items that you have not found, will be marked by a ?. This will be very helpful during the final stretch of the game, when you are trying to hunt down the rest of your items.

Here are a couple things to note when hunting your final items.

The first, is that there are several items located in save rooms. These can be tricky, as your map does not put a ? on that area, because the room is already covered by a save icon. I would recommend making a mental note whenever you notice a destructible door or opening in a save room, that you cannot explore at that time.

Second, is that there are also rooms with multiple pickups in them. The problem with these, is that they will show as cleared (represented by a dot) even when the second item has not been found. I only recall one of these such rooms, but there could be more (see the map for details).

Next, you cannot sequence break to get items if you want the achievement. Sequence breaking is the process of overcoming obstacles in ways not intended by the developers. If you get a certain item (e.g. missles) before you are intended to, that item will glitch and you will not be able to collect all of its pickups.

Finally, I would recommend finding all 100% of the items, before the explosion escape sequence. After this event you will no longer be able to reach the Pulse Rifle, if you don't already have it. It will also make navigation much more difficult, as you will trigger the final boss fight every time you move into the specific area. Although it is true, that you can explore after beating the game, you will still access the final fight every time you enter that area. This will make several areas hard to reach, as a big chunk of the map is inaccessible.

If you are still having trouble, here is a very detailed map. I take no credit for the map, as it was posted on www.Joystiq.com.

Full Map with Detailed Item Locations:

By far the best guide

http://www.achievementhunter.com/games/shadowcomplex/guide/

You won't miss anything with the guide.

New working link thanks to JMJimmy

https://web.archive.org/web/20141207123002/http://www.achiev...

Here is a complete guide to show you how to get it. It even shows pictures so you can get an idea of where it is.

http://guides.ign.com/guides/839087/page_6.html

For the Completionist achievement you'll have to finish the game with 100% of items in your backback. I thought whould be simple because of the "?"s on the map, but after I found every last "?" (I was level 30, so the entire map was visible) some items were still missing! WHAT THE... ???
It's just because 3 of them are not marked properly: hidden behind save rooms, so that on the map you can only see the little star (the save icon) and not the "?". They are an armour upgrade, a missile upgrade and a passkey (you need 12 to reach the Fusion Helmet).
The rest it's fairly simple to get: you just have to reach every "?" and try to figure out how to reach the hidden item. There are already great guides in this page, so you shouldn't have any problem if you need any more help. Just remember! if you're on the pier, you see the scene where the ship takes off and you're not 100% with items, reload the last checkpoint and go back to get what you missed. ;-)

NOTE: on INSANE you'll not see the "?" pointing the objects on the map. There's a little glitch, however: low the difficulty while inside your Insane game, "exit game" from the menu, and then continue from the last save. At this point you should see the question marks on the map. If you rise the difficulty back to Insane they won't disappear and you'll still have your x2 bonus at the end of the game. Easy!

Here the missing items:

I created a map that outlines getting 100% of items with a path to follow. They can also be used for Jason Bailey and/or ChAIRness master challenge runs.

http://picasaweb.google.com/bandito.lobster

credits:
Map: Somebody from somethingawful.com forums
Path: Q cumberR (mostly, I made a few minor tweaks)
Finished map: Bandito Lobster

http://live.xbox.com/en-US/profile/profile.aspx?pp=0&Gam...

http://live.xbox.com/en-US/profile/profile.aspx?pp=0&Gam...

Game Guide with videos

http://www.gamesradar.com/f/shadow-complex-100-guide/a-20090...

Here is another guide [only video] for all items.
Following this video you will complete the game at 100%

Let's Play Shadow Complex 100% Run
Author video is: Ramza411sb