Observatory
Upon entry, Curly will greet you with a cutscene about your final mission on the island. Speak with Balrog if you want to trade your Machine Gun back for the weapon you gave him before, then head to the door in the middle, on the left.
Plantation
Plantation is possibly the largest area in the game by size, but travelling to the top is fairly straightforward.
From the start, head across the bridge and all the way to the left of the map. There will be a sequence of rooms connected by pipes here. The 'right' pipe will let you travel to the next room in the sequence, while the 'wrong' pipe will send you back to the start. Most of the pipes have a straightforward path they'll follow out of the room, so it shouldn't be too difficult to figure out which one won't send you back to start.
After this small maze, you'll be on the second floor of the area. Head all the way to the right to reach a save room, then climb above it and head back to the left. A sign will hint that you'll have to fly between the horizontal Press enemies in order to climb to the next floor. As long as you remain centred between each set of Presses, you shouldn't be in any danger of getting hurt if you keep flying at top speed.
Head back to the left on the third 'floor' to a ragged bridge and another series of horizontal Presses. After this, head back to the right to the middle of the map to the final vertical section. You'll have to repeat flying between Presses, this time without a floor beneath you! If you mess up and need to fall, stay as centred as possible and your falling speed will be enough to avoid being killed on the way back down.
If you can reach the top, there is a Save Disk on the left, and the path forward on the right.
Last Cave
Upon entering the Last Cave, your weapon levels will be reset to 1. A major priority is building your weapon levels back up based on which ones you use the most. There are several Experience Capsules available during the climb through the cave that will help accomplish this goal.
The biggest obstacles here will be the spikes and red liquid, both of which deal 10 damage a hit. Your skills with the Booster v2.0 will be put to the test here.
On the second floor will be several red bat enemies that will fly to the right at varying heights. These enemies can be slain or avoided, but it's important to take your time.
When you reach the end of the third floor, be wary of the horizontal Presses. Remember to boost at full power as you pass them, but be on the lookout for a stray red critter that will try to tackle you on the way up.
When you reach the fourth floor...
BOSS: White Devil
The biggest challenge during this fight will be the shallow water on the floor. Try to stay airborne or near the edges of the water to keep your movement speed to a maximum so you can avoid its attacks.
The strategy for this fight:
After the fight, continue to the left to reach the exit...?
FINAL BOSS: JENKA'S NIGHTMARE
Genesis and Oblivion are back.
Your target is Oblivion. It will slowly travel to the left side of the screen before appearing, then attack you with an all-out barrage of different shots. Its only vulnerable spot is the eye itself, so you can only hurt it while it's attacking.
At the same time, Genesis will fly around next to Oblivion and summon various enemies depending on where you are. If you're right up against the floor, Genesis will summon red critters, but if you're in the air or on the platform on the left, Genesis will summon red bats. While Genesis seems to take damage, hurting it won't have any effect.
Once Oblivion's health gets low enough, it will open its eye into a gaping mouth and begin shooting charged shots at you. These shots hurt A LOT and are likely to level down your weapons almost instantly, so make sure to move out of the way when you hear it charging its attack.
The strategy for this fight:
When Genesis and Oblivion are defeated once and for all, your final destination is The Balcony.