Beginning of the Cavern
Start off by destroying the anklebiter generator to your right. Move to the left and open the barrel for a green potion. Kill a bomb thrower and the grunts past the barrel, along with a couple of generators. Open the barrel near the bottom of the area for a keyring. At the top, there are two chests you can open. The first and closest chest has a drumstick inside. The other chest on the left near the wall has a bomb, so ignore it. In the back are two barrels and some treasure lying on the floor. The two barrels have a cherry and junk in it. Walk to the left to pick up some purple crystals. Destroy some anklebiter and grunt generators in the area, and kill the archer shooting from the distance. The top left path has a barrel with a key inside, and a blue crystal behind it. The top path has a random chest. If possible, shoot the archer on the otherside as well.
Go to the left path and kill some more grunts and anklebiters, and destroy their generators, but be careful not to destroy the explosive barrel. Kill the lone archer and grab the turbo power-up next to the explosive barrel if you need it. Keep walking to the left, but be careful of the falling boulders. Grab the purple crystals and kill more grunts and generators in the next area. Grab the keyring and open the chest for a 3 way shot. Walk up the mini slope and kill more grunts, anklebiters, and generators, while grabbing some purple crystals next to the edge. Kill off some archers and bomb throwers in the distance as well. The bottom of the area has some junk, whereas the middle area has some other goodies instead. Next to the fire trap are two chests. The chest on the left is a random fruit chest. The chest on the right has a red death, so use a potion to turn it into an apple. On the right of the chest is a barrel with some treasure inside. In the back where the bomb thrower was is a barrel that has a pineapple inside. The chest to the right of it only has junk inside. Grab a blue crystal to the right, and ignore the barrel as it has nothing inside of it.
Middle of the Cavern
Go downward to find a flame potion at the edge. Open the barrel nearby to find a hint scroll.
This scroll is referring to the Javelin of Blinding, which we will be using for the Plague Fiend in the Sky Dominion. Walk on the upward path and collect the purple crystals as well, but be careful for falling boulders. Kill off some anklebiters and grunts, and destroy their generators. Kill off a bomb thrower near the top where the path will break off. Pick up the blue crystal on the left edge, next to the fire trap. Go on the right path and avoid the chest as it only has a bomb in it. Destroy the anklebiter and grunt generators nearby as well. Grab the flame potion and keep going upward. Destroy some more an anklebiter generator that is blocking the path, and grab the blue crystal too. Be careful of some more falling boulders, and try to kill the general safely. Grab the key he drops and deal with the grunts nearby, and destroy their generators. Before moving on, notice the fire trap at the top. To the left of it is a rock that is sticking out. Destroy the rock to reveal a secret area behind it, which has a switch that turns an island somewhere and makes it accessible, which is first of two steps to getting the Javelin of Blinding. The chest contains a super shot item.
Go to the right and kill off some more grunts and their generators. Open the chest near the edge for some treasure. Grab the lone green crystal near the unstable path that will break off as soon as you get near it. Follow the only available path and kill some grunts, anklebiters, and destroy their generators. Kill the archer behind a barrel if you didn't get him from the other side earlier. Open up the barrel and grab the key. Grab some purple crystals and open the chest near the anklebiter generator for a keyring. Keep going up and destroy the two red barrels to help take care of the grunts and their generators. Keep following the only available path while killing off some more enemies in the distance as well as their generators. Open the first barrel for a random potion, and the second barrel a little past it has a keyring inside. Grab the blue crystal and treasure and walk to the island that you turned with the switch from earlier. The chest at the edge contains some steak. The barrel only has some poison fruit, so avoid that and grab the ice crystal and key instead. The golden icon down below isn't raised, but we will be coming back for it very shortly.
End of the Cavern
Go back to the other side and continue following the path until you see a chest. The chest has a lightning shield inside of it, which will come in handy. Notice the rock that stands out on the wall to the left of the chest. Destroy the wall to find a switch that raises the golden icon we just saw earlier. Backtrack to the golden icon and it will raise a platform somewhere with a green crystal on it that makes the Javelin of Blinding accessible. Go back to where you used the switch and go downward to find some more grunts and an inactive golem. Kill the grunts first and their generator, then deal with the golem. Before following the path with purple crystals, use the switch nearby to raise a platform that will take you down below. Grab the green crystal we saw earlier and then grab the Javelin of Blinding. Now go and grab all the purple crystals that lead to the gargoyle. Kill off the gargolye and grab its golden icon, then deal with the anklebiters and their generator nearby. Open the barrel for a banana bunch and grab the one blue crystal nearby. Right next to the exit is the final hint scroll of the level.
The Castle Stronghold (which happens to be the next realm), contains the legendary item needed to do some extensive damage to the Dragon. Because we don't have the legendary item, and depending on your level, it would be wise to save the fight with the Dragon until much later on. So for now, just exit the final level of the Mountain Kingdom.