This is it, the final area of the game. That is, of course, unless you came here earlier with the Aqua Boots. I am assuming you entered from Crawky Forest. Your first goal is to find the Magic Mirror, which you will need to claim the Ascent Boots that will in turn allow you to find Haidee.
Conveniently, you'll find the Magic Mirror just east of the entrance, being sold for 25,000 gold. You'll need to loop around to the north to reach the cave. With this item you will be able to destroy Death Lords, including those guarding the Ascent Boots.
The Ascent Boots cost 70,000 gold, so you will need to save some money to be able to afford them. There is no particular need to get your Maximum Gold above 72,000, so it isn't a bad idea to save your money by not buying any Bibles you come across once you reach that threshold. You will still want more Life Potions, though, so head south to find the first of the two hidden in Haidee's Forest. You'll need to destroy four Blue Vortexes from the west side of the trees. There is one last Life Potion to find, but you'll need the Ascent Boots to get it.
While you can explore the rest of the forest at this point, it is much more productive to do so after you've bought the Ascent Boots. The best thing you can do now is start farming gold. The most efficient farming location is still the White Rats in Crawky Forest, but if you want to stay in the area, try fighting enemies in the vicinity of Randar, on the east side of the northern ridge. Your best bet is probably the White Trolls directly north of there. You will have to leave the screen repeatedly to reset their spawning when they appear out of reach, though.
Once you have at least 68,000 gold, follow the western path to its north end. There you will have to defeat four Death Lords (and earn 2,000 gold) to reveal the cave where you can purchase the Ascent Boots. With these you can go practically anywhere, to the point where it almost seems like a hack. You can go fight Haidee now, but you may want to collect a few stray items first.
The last unique item you can find is Ramurasu's Pendant, which is for sale for 25,000 gold back in the middle of the chasm in the Graveyard of Bachular. Fortunately, there is a shortcut available to get there quickly—travel to the northern lake and follow the river east. You can follow that river all the way to the cave. Ramurasu's Pendant will decrease damage done to you by half, replacing the 1/4 reduction of Aresta's Pendant.
The final Life Potion is located in the southeast of Haidee's Forest, and is sold for a very reasonable 5,000 gold. (If this is your 19th Life Potion, a bug will significantly change the effects of Mea for the rest of the game.) You'll also find the final three Bibles in the southern part of the forest, if you want to cap off your Maximum Gold. The last item of note is the final Ring, located just southwest of the last Life Potion. Give it to a wise woman northeast of the entrance to Haidee's Forest to learn the hiding place of Golvellius. You'll still need the seventh Crystal before you can enter, though.
Once you're done exploring, it's time to take down Haidee. Head for the island in the middle of the southern lake. With the Ascent Boots, you can reach the rocks that you must destroy to reveal the entrance to Haidee's Cave.
Haidee's Cave
This is the final vertically-scrolling dungeon in the game and, as you might expect, it is filled with dead ends and obstacles. The dungeon splits right away, eventually resolving to four paths. The far left and middle right are safe. Head right immediately after those paths to avoid another dead end. You'll have to contend with a Giant Bat spawning multiple Bat Minions while you weave your way back and forth through sharp turns. Stay near the top of the screen once the Giant Bat is defeated so you have plenty of time to react. After a lull, you'll face the toughest turn in the game, so be ready.
You won't face any more dead ends, but it's best to bear left when two sets of paths cross in an X. If you're not distracted by the hordes of enemies in the final area, you'll note that the blocks which are trying to hard to block you spell out "HELP!!"
Compared to Crawky, Haidee is a pushover—assuming you have a lot of Life Potions and Ramurasu's Pendant, at least. Haidee will alternate between chasing you and standing still to summon homing projectiles. You can destroy the projectiles with your sword, but if you have reliable defenses, you can win the fight by simply taking the projectile damage and attacking Haidee directly. You'll still want to get out of the way when Haidee starts moving, but he should fall before you do.
At this point, you are filthy rich. You'll need to pick up the final Crystal (and any others you may have skipped), which will cost you 40,000 gold. Any excess can be spent on any items you missed, or new Mea. The Crystal is found in the northeast corner of Haidee's Forest.
With the last Crystal in hand, it's time to fight Golvellius. The wise woman's hint points to a location in Crawky Forest, so head back that way.