You can replay the first stage to gather resources, exp, and money. Grind out money for the shotgun. 2nd stage enemies are mainly weak to fire, so bring Molotovs and learn the fire skill for the shotgun. Bring Sedatives as well, the snake enemies can paralyze you.
I went back and replay the game from the start. The other important parts, that I didn't realize because I'm playing in Japanese, is that the treasure chests are very import. They drop documents, with perk skills that you can Learn, which really make a huge difference.
Also, the attack bonus you can pick up from that random vending machine for 100 credits or health is very important.
The snake enemies are really annoying, if there are more the 1 on screen, I found the best defense is throw a Stun Grenade. Always take them out first.
I pick up the shot gun and used the speed boost ability 20 SP from the hand gun, before switching to it. Make a HUGE difference when the bossing is really fast. Also, the Slow Shotgun ability can help even the odds as well.