Watch a short video walkthrough of this stage.
This stage is really nothing special. Attack the Blue Thieves for six magic pots, then continue onwards for a miniboss fight against General Heartland backed up by a Red Heninger and Red Longmoan. Since they're just stronger versions of their usual selves, take out the weaker ones first then concentrate on the giant. Don't use magic here as you need it for Death Adder himself, who is up next.
In the arcade, snakes slither together to form Death Adder. In most home versions he merely comes out of the double doors. Fortunately, in the arcade, this stage is part of stage 5, which means you get to keep your blue dragon. Either way, this is a difficult battle. Do your best to save your magic until skeletons start showing up, but use it if it seems like you are in danger, or immediately if you get knocked off your dragon to make sure you can get back on safely. Next, finish off all remaining enemies. In the home versions there are two White Skeletons, while in the arcade there are four Purple Skeletons. You can now focus on Death Adder. He is much like Death Adder Junior from Beginner Mode. He has a habit of throwing out sparks of lightning along the ground if you get a certain distance from him. Jump over them if you can, and if you are lined up with him, take the opportunity to perform a downthrust while he's stuck crouching for Massive Damage. Note that the sparks pass harmlessly through enemies so you can't simply get behind a foe to escape them.
Death Adder is vulnerable when he crouches to use his spark--you can get a few hits in during this time. When he gets up he will often use an attack immediately, so try to stay clear.
If you are experienced in the jumping slash, you can time your jumps so that you are above the level he swings his axe, and slash him in the head for an instant knockdown. Then yo ucan jump as he's standing up and repeat the slash. This is very useful in the arcade version.
In most home versions, once you are down to just him, you can cheat. If you swing slowly enough, you can attack him and he will not fall over until he has taken lethal damage. This works on all enemies if you can get that first swing in.
Your character will walk over to one of the pillars and the King and Princess are lowered from the ceiling. In the arcade, they thank you and it's all over; you then get to see your Score. In the home versions, however, they tell you Death Adder may have been only the servant of some greater evil, and that he sometimes went through the big doors. You head on through them.