For the most part, Super Mario Galaxy 2 is a pretty easy game. You run around in a beautiful 3D world following the stars. If you come across a difficult enemy, most of the time you can just run away. Sure, you may die a few times at a difficult jump, but you will always have a huge stockpile of extra lives to see you through to the end.
Before a hard boss or a hard task, it can be a good idea to grab a few extra lives. You can do this in many ways: collecting 100 star bits, collecting 100 coins, hitting that 1up dice on Starship Mario, and talking to the mail toad. If you notice a particular level that is pretty easy to do and will give you a lot of star bits or coins, make a mental note and you can always come back to grind out a few extra lives. Also keep track of where 1up mushrooms are on short levels as well as those arena warps where you have to defeat a bunch of enemies to get three 1up mushrooms.
Your co-star can be extremely helpful so don't be afraid to ask someone to jump into a game with you, even for a few minutes. They can pick up 1up mushrooms for you and also grab small annoying enemies and keep them out of your face. Shooting star bits out isn't as useful, but the co-star's ability to grab coins is incredibly useful, since coins give you back your health if you get hurt. During a boss battle, you can focus on your fight while your co-star shoots out a coin and grabs it for you to keep you on top of the game.
Most boss battles requires you to repeat a certain attack at a certain weak point around three times. Do not forget that your ground pound (Hit Z in the air) lets you drop to the ground quickly and dodge airborne attacks. If your enemy is chasing you and you don't think you'll get away fast enough, do a long jump (run, then hit Z and A quickly) for an emergency getaway. Otherwise, take your time to figure out how to beat the boss and don't panic. If you get stuck and keep losing lives, the cosmic guide will ask you if you want to let it show you how to win. Let it play for you and you'll see how to beat the boss later yourself!
When you are on Yoshi, you should never have to be afraid of falling! Super Mario Galaxy 2 has a glitch where you can do infinite flutter jumps with Yoshi. In order to do this, all you have to do is press and hold A to jump and flutter. Before Yoshi finishes the flutter jump, quickly let go of A, push and let go of Z and push and hold A again. You must do the A to Z to A sequence very quickly in order to get another flutter jump. Do it again before the flutter ends and you'll be able to chain the flutters together and go just about anywhere you want! You can easily practice this move on Starship Mario after finding Yoshi. Practice both fluttering up high and also fluttering across a distance so that you can do anything.
Near the steering wheel on Starship Mario, there is a Banker Toad that will hold onto your star bits for you. If you give him enough, he will start wearing some weird outfits and eventually unlock a new level for you!
To unlock Luigi, defeat Bowser at Bowser's Galaxy Generator and then talk to the Mail Toad. She will give you a message from Luigi and you will be able to see a "L" on Starship Mario. Go inside and you will become Luigi and you will be able to play on any level as him! After you beat the game, you will also unlock the Secret World with some more levels. If you have collected all 120 stars and beat Bowser again, you will get treated to a cutscene of Rosalina (from the first Super Mario Galaxy game) reading to the lumas about a green star.
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