1st Costume - Unlocked
2nd Costume - Unlocked after 30 kills
3rd Costume - Unlocked after 175 kills
4th Costume - Unlocked after Sim Mission is complete or 250 kills for DLC Characters
Play through the game once on Normal/easy and unlock as many costumes as you can (1-2 heroes at a time). I went down the roster left to right and got all the way to Elektra on 1 time through the game DO NOT PLAY ANY SIM MISSIONS UNTIL AFTER DOOM IS DEFEATED. This prevents costumes from locking and you needing to start over. After you beat Doom you can warp back to any previous HQ and then play the missions. You only need bronze on every mission to unlock Silver Surfer.
Simulation Disc Locations:
Wrenchworks - Captain America
Project Labs - Wolverine
Iron Man Lab - Iron Man
Temple of Negrete - Mr. Fantastic
Golden Court - Thing
Forbidden Passage - vs Crimson Dynamo
Sanctuary - Dr. Strange
Fake Doom Castle - Mysterio *** (you cannot re-extract back to this location once you leave sub-level)
Big Top - Luke Cage
Maze-O-Death - Elektra
Realm - Blade
Sepulcher - vs Scorpion
Stygian Abyss - Ghost Rider
to Bridge/Bifrost bridge - Storm
to Bridge/Midgard gate - Spider-Man
Camp/Warrior hall - Thor
Asgard City/Grand Hall - Deadpool *** (you cannot re-extract back to this location)
Asgard City/Great Forge - Invisible Woman
Shore of Corpses - vs Shocker
Spire's Ascent - Spider-Woman
Fighter Bay - Iceman
Hyperdrive - vs Ultron
Outer Hull - Ulik
Sky Towers - vs Piledriver
Cityscape - Ms. Marvel, Human Torch
Heights of Progress - vs Dragon Man
Above all: To get this achievement you need to unlock Black Panther and Daredevil by finding 5 of their respective 8 figures in the game. Upon collection of 5 figures for each their simulator mission is immediately unlocked.
I wouldn't recommend playing any of these until after you've finished the game on easy or whatever difficulty you're on to prevent costumes from locking.
To get a few more achievements, don't use the first or the one before this and to the following:
After you have beaten the game on Normal, start a new game on Easy and when it asks you choose "Load game statistics" and load your last save on whatever difficulty you want to load. This will make the Sim missions easier. Now get the rest of the costumes (if you haven't already) and grab all of the discs but do not play the missions until you have beaten the last fight against Doom. This stops the glitch from locking costumes and it makes the Sim missions that much easier because your characters are at least level 45-50. After you get ALL the costumes, play through the simulator missions, saving after bronze or above ON EVERY MISSION (Gold is very easy but you may have to restart a couple of times to get the Golden Age of Comics achievement). If you follow this you'll have unlocked Surfs Up, Golden Age of Comics and Dressed for Success.
If you think this solution needs to be edited/updated. Leave a comment and please explain how I could make it better.
Edit 2/20/11:poordude1089 cannot re-extract to Deapool's disc location