Go to the Alkali Lake section of the Frozen Tundra (before the Agent Zero fight) and find the fallen tree just past the long road section with the shield troops. Enter the forest that leads to a cave to find a sword and skeleton. Collect up the mutagen.
Usually, using cheats disables the ability to unlock Achievements. To get around this, enter the desired cheats, then quit to the main menu. From here, select "Continue" or "Replay Mission." Your cheats will still be in effect and Achievements enabled.
ACHIEVEMENT - HOW TO UNLOCK Getting Started (10 points) - Kill 100 enemies. A Day's Work (20 points) - Kill 500 enemies. What I Do Best (30 points) - Kill 2000 enemies. You Can't Hide (20 points) - Lunged to 250 enemies. Lunge (10 points) - Lunged to 25 enemies. Pounce (15 points) - Lunged to 100 enemies. Piggy Back Ride (10 points) - Lunged to a W.E.N.D.I.G.0 prototype's back. Quick Killer (10 points) - Quick Kill 1 enemy. Efficient Killer (15 points) - Quick Kill 20 enemies. Perfect Killer (20 points) - Quick Kill 3 enemies in a row. Drop Dead (10 points) - Kill 10 enemies by throwing them off high areas. Apprentice (10 points) - Raise One Combat Reflex to Master Level. Samurai (25 points) - Raise All Combat Reflexes to Master Level. Mutant Lover (15 points) - Raise one Mutagen to level 3. Astonishing (20 points) - Found 1/2 of all Dog Tags in the game. Devil's Brigade (30 points) - Found all Dog Tags in the game. Defensive (10 points) - Performed 1 Counter move. Untouchable (20 points) - Performed 25 Counter moves. Catch! (10 points) - Kill 1 enemy with a reflected projectile. Boomerang (20 points) - Kill 25 enemies with a reflected projectile. Aerial Assault (10 points) - Performed 10 Air Grabs. Ultimate Wolverine (15 points) - Fought 4 W.E.N.D.I.G.0 prototypes at the same time and defeated them at Alkali lake. Hot Potato (20 points) - Light 20 enemies on fire. Shotgun Epic Fail (15 points) - Kill 25 Ghosts with their own weapon. James Howlett (15 points) - Performed a Wolverine to Wolverine Lunge. WoW! (15 points) - You feel cold as you examine the skeleton and read the name "Arthas" etched into the nearby sword. Aerial Master (15 points) - Got 6 enemies airborne at once. Fully Loaded (35 points) - Maxed out all upgrades. Slice n' Dice (15 points) - Kill 6 enemies with a single attack. Found! (15 points) - You found a mysterious hatch. Slaughter House (15 points) - Dismembered 100 enemies. Blender (25 points) - Kill 200 enemies with Claw Spin. Walking Death (50 points) - Beat the game on Hard Difficulty. Heightened Senses (20 points) - Kill 200 enemies in Feral Sense. Environmentally Friendly (15 points) - Kill 10 enemies using objects in the environment. Whatever it Takes (20 points) - Kill 30 enemies using objects in the environment. Bloodlust (20 points) - Kill 50 enemies while in Berserker mode. Weapon X (25 points) - Kill 150 enemies while in Berserker mode. The Cake (15 points) - Find the cake. Bar Fight (30 points) - Defeat Victor Creed (Sabretooth). Spillway Escape (30 points) - Escape from Weapon X. Helicopter Ride (30 points) - Defeat David Nord (Agent Zero). The Village (30 points) - Complete the Jungle Mission. Put up your Dukes (30 points) - Defeat Fred Dukes (The Blob). The Dead Pool (50 points) - Defeat Weapon 11 (Deadpool). 52 Pickup (30 points) - Defeat Remy LeBeau (Gambit). Threading the Needle (15 points) - Flew through the O ring in the Sentinel Boss Battle. Fallen Sentinel (30 points) - Defeat the Sentinel Mark. Stick Around (10 points) - Impale Victor Creed outside the Bar. Clean Up on all Aisles (10 points) - Destroy all objects in the grocery store fight with Fred Dukes (The Blob).
Unlock the following costumes by performing the corresponding Challenges:
COSTUME - HOW TO UNLOCK Blue/Yellow Wolverine costume - Collect the two action figures of classic wolverine. Then, complete classic wolverine challenge. Brown/Yellow Wolverine costume - Collect the two action figures of legendary wolverine. Then, complete legendary wolverine challenge. X-Force costume - Collect the two action figures of x-force wolverine. Then, complete x-force wolverine challenge.
After crashing the jeep on the first level and proceeding, you will reach an area with a barricaded gate. Climb over the rocks and go in. After taking out the enemies, search for some boards over a small doorway off on one of the areas. Slice them up and enter to find the hatch on the ground near the back corner.
On the first level at the Gatecrasher objective, crash the jeep through the gates killing all enemies. To the left of the gate there is a boarded up door. Cut the boards down, and enter it to find the mysterious hatch.