Get Vulcan Fury:
After suttle underground fighters go talk to Torn and he will say here a vulcan barrel for that pee shooter of yours which is blaster weapon. 

How to Get Special Technique Dark Bomb:
After you collected 25 metal head skull gems go to the oracle and this icon guide you there 

Secrets:
Collect the following number of Orbs to unlock the cheat indicated in the Secrets Menu:

Collect 5 Orbs - Toggle Jak's Goatee
Collect 15 Orbs - Mirror World
Collect 30 Orbs - Big Head Mode
Collect 45 Orbs - Small Head Mode
Collect 55 Orbs - Scrap Book 1
Collect 65 Orbs - Scene Player Act 1
Collect 75 Orbs - Vulcan Fury Course
Collect 85 Orbs - Scrap Book 2
Collect 95 Orbs - Scene Player Act 2
Collect 105 Orbs - Peace Maker Gun Course
Collect 115 Orbs - Scrap Book 3
Collect 125 Orbs - Scene Player Act 3
Collect 135 Orbs - Reverse Races
Collect 145 Orbs - Level Select
Collect 155 Orbs - Unlimited Ammo
Collect 165 Orbs - Unlimited Dark Jak
Collect 175 Orbs - Invulnerability
Collect 200 Orbs - Hero Mode 

Alternate scrap book: 
To get this you must collect 200 orbs in Hero Mode

Gun Locations:
Morph Gun: Scatter Gun. You get this on the mission for Krew where you go with his bodyguard to take out metal heads. 
You have to complete the shooting range first though.
It is just like a shotgun. Wide range, close combat etc.

Morph Gun: Blaster. Again given to you by Krew. This is a long distance, single shot rifle, 
with decent firepower and laser sighting. You get another shooting range with this.

Morph Gun: Vulcan Cannon. Yeah Baby!! This hell of a upgrade can take down a wave of Krimson Guard 
before you can say, "Praxis is a " Hold down R1 to turn this baby into a Gattling Gun. 
Watch your ammunition supply though!

Morph Gun: Peacemaker. I Haven't got this mutha but it does more damage than crashing a Hellcat Cruiser
(These go down really easily!)
I'm going to keep searching though.
Praxis battle: This is the one on the palace rooftops. Its not hard but its not easy either.

You can get the peace maker after you escort Krew's men to the sewers. 
It is the darn best gun ever! To find it, go to the shooting gallery.
 - Mr.X

First get your blaster and take any available shots, without getting hit. 
When Praxis recharges get out the scatter gun and jump+shoot. Repeat until yellow health. 
Then Praxis fires bombs. Shoot a t him with blaster while dodging. When on red health, 
avoid tornados and keep shooting. when he charges at you roll then shoot. 
It'll be over before you know it!

Complete securi-tank mission easy
I spent about an hour trying to beat this mission and I came out with a pretty good strategy. 
In the first room it trys to shoot you so move left and right alot. 
Then when it breaks the wall start doing LONG JUMPS. 
When you roll press the jump button and you'll go far, fast. 
This method will get the tank off your back. 
Then use the bar to swing into the third room. 
use the platforms to destroy the things in the middle 
then move on to the next room. It is pretty easy there on. 
and in the end just let the tank destroy the barrels.