Batman can find hidden envelopes throughout the game: the first is on the Rooftop Battle level, the second is on the Bridge level, the third is on the Plant Electrocution 2 level, the fourth is on the Gasworks Level, and the fifth is on the SWAT Chase level. If you find one, it will have a letter on it. They unlock infinite weapons, charge bar, and health as well as double damage, and invisibility. They are awarded by the order found, and not the letter. For example, if you collect Envelope E furstm you will get infinite weapons instead of invisibility.
Envelope B on Bridge:
Knock out the thug at the beginning of the level. You will see the glowing item on the left when you start. Climb the ladder and jump/glide around the crate onto the other crate, then glide onto the platform with the letter.
Envelope E on Gasworks:
You will find a room with two levels, each floor with two goons on them. First, take out the goons and look around on the lower floor until you see a sewer pipe with boxes in the sewage. Carefully glide down and land on the first box. Jump over the next few boxes and pick up the envelope with an “E” on it. Complete the game and the “Infinite Health” cheat will be unlocked.
To fly, press JUMP then press B twice. You cannot increase your altitude or fight while in the air.
During the level after the Batcave training, look around the big square when you get to the area with a Joker thug. You will eventually find a ladder. Climb up the ladder, then look around for a box. Pick it up to get smoke bombs.
Hacking into Issac Evers files:
Find the second wire of the color you need (that is not powered). You must make the powered wire reach the non-powered one. For example, if you work on blue, you need to connect the live blue wire to the dead one.
Look in your inventory and examine the disc Batman picked up before fighting Mr. Freeze to reveal possible colors that work with the Blimp puzzle. The combination is Purple and Green. Make the color purple, “mix tubes”, then make the color green.
Mr. Freeze strategy:
Do not charge at him or you will be thrown and likely get frozen. Enter first-person mode and equip your demolition charges, then make sure Mr. Freeze is following you. Look up to find glass tanks moving on a hook. When he is under one, fire at it with your charges then detonate it so that it lands on him. Repeat this until you get a move where Batman grabs Mr. Freeze’s gun and freezes him to stop him from burning.
Joker’s thugs strategy:
You must be fast to knock out thugs after the Joker’s death. Use your block once your attack is over. One combo is to go back a few steps, move forward and punch, then quickly kick two times. This is useful when knocking thugs out while they are blocking your attacks.