Note: This will be useful only if you start the game, also this might execute a glitch that causes a piece of equipment be lost and another rifle will be bought automatically, this will give you another rifle.
Step 1: Go to equipment and choose Alien Pistol
Step 2: Go to level 1, then drop the gun and get the Pistol CS-Pro
Step 3: Complete the level
Step 4: Go to equipment, and exchange the Pistol CS-Pro for the Alien Pistol
Step 5: Repeat and upgrade your equipment.
This way you will get a small amount of credits (150), but it is a fast way to get better equipment if you repeat the process a lot.