Locating the Zone entrance[edit]
Your next destination will be a similarly easy to reach room which contains a zone entrance that you already passed. Back when you visited Room D to reach Room E and Zone 3, you skipped a more difficult zone in the process. From Room A, walk to the left to reach Room D beyond the fireplace, but don't forget to stock up on supplied if your low on ammo or health.
Room D[edit]
Since you've already seen this room, there should be no surprises. If you have enough health and ammo, then you're all set to go. The gun and potion on the East and West sides of the room respectively should help. Enter through the door to visit Zone 6.
Into Zone 6[edit]
Zone 6 isn't quite as water logged as Zone 4 was, but you'll still need the Air Helmet if you want have any hope of surviving it. Because it is not entirely aquatic, you'll have to face more enemies than just fish here. Winged demons routinely make appearances throughout the stage, as well as the occasional bat. When you reach the middle of the map, be sure to take the upper path to avoid hitting a dead end.
Your gun will be your primary means of defense throughout this zone. You can rely on your knife if you prefer, but the fish and demons are capable of making sudden movements that can jeopardize your health if you're not lucky and get attacked before you can strike. You can also rely on the strategy of tossing a grenade if the screen becomes occupied by more than one demon to quickly eliminate them at the cost of a little health.
Boss[edit]
The boss of the sixth zone is surprisingly easy. It slithers across the floor of the room occasionally shooting a worm like projectile at you, which moves at a fairly slow pace. In order to defeat the boss, all you have to do is hang back on the left side of the floor and fire away until the boss runs out of health. The caveat to this fight has to do with the projectile that the boss fires.
Inconveniently, it happens to hover just above your line of fire. So if you do not move, it will draw close enough to attack you. Your best bet to deal with it is to wait until it is a little more than half way to you, and then jump up and quickly fire as soon as your feet leave the ground. You should be able to remove it. Then continue to fire at the boss and repeat the process if it launches another projectile at you. Defeat the boss to collect yet another red health pill that will add another 100 points to your life, as well as the sixth laser gun piece, and head back to the zone entrance to continue your search.