To get to levels 2 or 3 with maximum weapons and lives use these codes.
2- "CAAEIEAAAUABHGADCKBX"
or "CACGFAAAAUAEDIADCKCY"
3- "DAAGJHCAAUAEGAADCFCU"
or "DACIHAFAAUAIGAADCDBN"
First off, collect the necessary objects from the desert screens. The objects required are the GUN, AMMUNITION (FOUND IN LARGE BOX), ACID BOTTLE, KNIFE and a WATER BOTTLE for use later on.
On the desert screens if you're hit by whirlwinds then shake the joystick left and right before your energy is sapped. To collect the water bottle first find the knife and the water bottle and ensure that both are displayed as icons in the objects/ weapons display. Now stand left or right of the base of a cactus and stab it to replenish your rapidly dwindling water supply.
Now enter the cave entrance and enter the first lead-off cave. Inside is a chest which needs to be kicked or punched (keep trying both) within which is a book revealing the details of the quest ahead.
Enter the water section and use the knife to kill the alligator. Enter the end cave beyond the water section and collect the gold nuggets and machete (the gold nuggets need to be collected to complete the game). If you are using the book, then standing in front of the wall will display all of the screen which must be crossed to finish the level.
Exit from the cave section and use the machete to remove bushes from in front of the jungle section. Enter the jungle section and move right while avoiding the wandering natives and monkeys up the trees.
You'll now come to the final screen where you need to use the acid bottle on or near the chains tethering the giant slug. The slug will be released and unsurprisingly will kill you if you stand in it's way. Shooting the slug causes it to strobe between skeleton and normal but it cannot be killed.
Exit the level through the to right corner of the screen.
...x shield on going down as it is very difficult to get through without it. Watch for the three-way bullets and stay between the middle and lower ones.
The underground section is easy enough with no real surprises other than the odd enemy above. When you finally reach the exit and the bouncing monster, shoot it quickly from behind as you follow it. As it does a high bounce, go over its head and shoot it from the other side, repeating it on the other side. Simple.
Same as the other levels and you must keep on the move. Be sure to pick up the blue flashing thing (whatever it is) between the towers; it gives you an extra man, full energy and acts as a smart bomb. Collect the double shots and missile to dispose of the end-guardian with ease and then take on the Muddy himself (itself ?). Surprisingly he is the easiest to dispose of by staying at the bottom of the screen where the two bullets come out. Hover between these and keep firing, the missiles will do the rest.