
During the boss battle with the Sentinel, while falling back to earth, destroy the second turbine on the Sentinel's back and a large O-ring will fall off. Fly through it by pressing X at the right moment to get the "Thread the Needle" Achievement.
In the first fight with Sabertooth, drain his life twice and the fight will continue outside the bar. Outside there are some spikes on the walls (you can spot them using your Feral Sense). Grab Sabertooth or hit him to send him flying into one of the spikes to unlock the secret Trophy "Stick Around".
In the battle with the Blob, lead him around, destroying every single element of the grocery store to unlock the "Clean Up on All Aisles" Trophy.
Unconfirmed: Enter these codes during gameplay for the desired effect. A sound will notify you if the code has been entered correctly.
Outside the Akali Lake Weapon X facility (the "snow level") you'll follow a narrow path. Look for a log leading up to a cave. Inside you'll find a sword and with a peculiar inscription. You will unlock the "WoW!" Trophy by finding this hidden area.
In the first level, just after you crash the Jeep, you'll have to climb some rocks and fight off a wave of baddies. Look for a boarded doorway. Slice and dice the boards and follow the passage to a hidden area with a hatch which may be familiar to Lost fans. This unlocks the "Found!" Trophy. Wonder what Desmond's up to down there?
In the Sentinel Factory you'll have to lower your body temperature to avoid a heat sensor. When you have to move the teleporter, move it to the far right. It will reveal a secret room through the wall. And there will be cake. Celebrate your triumph with "The Cake" Trophy.
Beat the game once to unlock Hard difficulty.
Unverified: Collect all three figurines in each level to unlock additional costumes. You'll have to fight a version of Wolvie in the costume (in a bonus level) before it becomes available to select at the beginning of a level.
Unverified: Beat the game on Hard to unlock old-school 3D mode (you'll need some red/blue glasses *free with Rad Racer!).