Ultimate Code:

For level select, all weapons and much more, enter the code
NTHGTHDGDCRTDTRK at the "Enter Cheat" menu. 
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Results:                 Actions:                        

 All Weapons        -->   CMGTSMMGGTS
 Dana Mode          -->   DNCHN
 Disco Mode         -->   SNFFRR 
 Gallery Mode       -->   THBST
 Greg's Mode        -->   GRGCHN
 Pen and Ink Mode   -->   DLKTDR 
 Infinite Lives     -->   FRTHSTHTTRLSCK
 Spirit Mode        -->   THSSLKSCL
 Robin's Cheats     -->   RBNSMTH
 Show Credits       -->   FDTHMGS 
 Unlimited Ammo     -->   BLLTSRRFRND 
 Ultimate Cheat     -->   NTHGTHDGDCRTDTRK
 Big Heads          -->   TSHNTTBNCTPRDCRD
 Purty Colors       -->   LLTHCLRSFTHRNB
 Quack Mode         -->   CLLTHTNMTN
 Show All Enemies   -->   NSTHMNDNT
 Show the Credits   -->   FDTHMGS

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Beat the Campaigner:

 When  facing the  Campaigner  (the final boss), take out your Particle
 Accelerator  and  start  charging it.  As you charge it the Campaigner
 should  stall  and  stand still.  Now fire it at him and  change  your
 weapon (the minigun and auto shotgun work well.)

 Even though he has a forcefield he'll start laughing as he blocks your
 attack.  This leaves him  open  for your  second  attack.  Repeat this
 process for an easy victory.

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Grenade Launcher in the Ruins:

 When you get to the temple with the save  point on top, save then look
 around.  Look to your side and you'll see a ten-point Life Force. Walk
 toward it and you should see a s parkling  path.  Follow it and you'll
 find the Grenade Launcher. 

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Jumping from Pole To Pole:

 An easy way to solve that is by  putting  on the map. When you jump as
 soon as the map  arrow (meaning your position) goes over the rock pole
 marked on the map let go of the  button and you will fall right on the
 rock pole.  This stops you from falling when jumping from rock pole to
 rock pole. 

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Ultimate Code:

 For  level  select,  all   weapons  and  much  more,  enter  the  code
 "NTHGTHDGDCRTDTRK" at the "Enter Cheat" menu.