It's finally time to restore light to the ORB of Earth. With the ROD in hand, you need to return to the Earth Cave. You will have to travel the same path as before, dealing with the same enemies. Hopefully you already collected all the treasures, so you don't need to spend any time or effort doing that. When you reach the VAMPIRE's room, head north and use the ROD to open the way to B4.
Earth Cave B4
The denizens of B4 are much the same as those on B3. You still need to be careful of COCTRICES, but the primary challenge on this floor is navigation. Like B2, the floor consists of many interconnected room-like areas. There are two treasure rooms to be found here, each with a variety of guards (but fortunately no more EARTHS.)
Heading north and west from the start will lead you to the first treasure room, which contains five treasure boxes. You'll find over 11,000 G here, as well as a worthless Wooden Staff. There are two points where you will run into a combination of a GrOGRE, a WzOGRE, and as many as two HYENAS, directly below each of the two chests on the right side. Approach these chests from the sides (or between them) to avoid these battles. If you do fight, take out the WzOGRES quickly. Their attack isn't particularly threatening, but they will cast a series of debuffs and, if the battle drags on long enough, ICE2. However, they will always start battle with RUSE, which boosts their Evade to almost unhittable levels. A concentrated attack should be enough to take them out despite this. Alternately, use MUTE to negate the threat and deal with their allies first.
There is another guard point in this room, this one containing one or two SPHINXES. You will trigger this battle by approaching the southwestern chest from the east or north, but it can be avoided by following the bottom and left walls around. SPHINXES have a lot of HP and three dangerous attacks, but they also have very high Evade and Magic Defense. There really isn't any good way to defeat them (especially since the LOCK spells don't work), but a sustained physical assault will (eventually) get the job done.
Once you finish with that treasure room, head back toward the entrance to the floor and instead travel southwest. The path branches, but both directions lead forward provided you keep pressing west. The stairs to B5 are in the northwest corner, while the second treasure room is to the southwest.
This treasure room contains a CABIN, 1,250 G, and a nice Silver Shield for your FIGHTER. There are three guard points featuring the same WzOGRE encounter from the first room, but all of them are avoidable. They are found east of the southwest chest, south of the east chest, and south of the north chest.
Earth Cave B5
The final floor of the Earth Cave contains no treasure. The quickest route to LICH is to head north and east, then swing back south as soon as you can. Keeping the path short will keep the encounters to a minimum.
The enemies found on this floor tend to dish out a lot of damage, but you won't find the annoying WIZARDS or COCTRICES here. You will find EARTHS, albeit rarely. When you are ready, approach the glowing ball in the western room to begin the fight for the ORB of Earth.
LICH
Now this is a boss fight. Although the battle music is the same as always, you'll notice that you no longer need to select a target for your attacks. Also, LICH is huge compared to other enemies. This is not an easy fight, but it is winnable if you go in with high HP. LICH will cast spells 3/4 of the time, and will open the fight with ICE2. You can try casting AICE in the first round, but there's a good chance LICH will get his spell off first. Luckily, his next few spells aren't very dangerous.
In the early going, try to cast FAST on any warrior who will be attacking LICH with weapons. He has 40 Absorb and 400 HP, but you can cut through that pretty quickly with some timely critical hits. LICH's attacks cause Paralysis, which you can't do anything about, so try not to rely on a single character for all of your offense.
LICH's Magic Defense is quite high, but he is vulnerable to HARM and Fire spells. Every little bit helps, and you can teleport out of the Earth Cave after the battle, so don't hold back. He resists Status, so most debuffs (including MUTE) are out of the question.
If the battle wears on, be prepared with defensive spells. After SLP2 and FAST, LICH will cast LIT2. Cast ALIT after you see SLP2, just to be safe. Follow that up with AFIR, because LICH will continue with HOLD and then FIR2. If he gets around to casting SLOW, you may need to recast FAST, but by this point he should be low on HP. If he isn't, your chance of victory is pretty low.
With LICH defeated, walk to the back of his room to be transported out of the Earth Cave. I recommend saving immediately, then patting yourself on the back for restoring light to the ORB of Earth. One down, three to go!