There's nothing to do outside, just head on in.
Round 1: 3 Golems[edit]
If your character has a long range attack, fight as far away as possible, so that only one of the golems has you in its line of sight. Enemy fireballs split and bounce off of walls... you will be doing a lot of dodging in this fight. The fire darts are faster, but a lot easier because you only have to dodge them once. Attack from a diagonal, because you want as much screen space between you and them as you can get, so that you get plenty of warning when one of their fireballs streaks toward you.
Climb up the castle, and go onto the next fight.
Round 2: 4 Spiders[edit]
These monsters will try to entangle you by spitting out web fluid. Fortunately, this only goes in 2 directions, so stay away from the bottom diagonals. They also have a non-entangling missile attack that goes in all directions, but they still rarely choose attack the sides. Try to bunch them together, so that by staying above one you stay above all. You're in way too small of a space to dodge attacks from every direction. Attack them from the top, and when they get too close, circle around and lure them down. Rinse and repeat.
You can defeat the first spider easily by trapping it on one of the corners near the entrance and then firing away. This also works if you manage to lure other spiders into that spot. Cheap, but it makes things a heck of a lot easier.
Round 3: 4 Worms[edit]
You can only hit them when they are out of the ground. The worms throw rings which will speed towards you no matter what direction you lie in. If you ever collide with one of them, you will get hurt, so try to stay below or to the side of them if possible. Keeping to the bottom wall also means that you don't attract the attention of those that are further up until you're ready.
Round 4: ???[edit]
Whoever you chose as a companion back in act 2 will be here, having given him/herself over to Casomira. You will have to kill him/her. Kahdega and Nadiru are pretty straightforward, like the prison boss they put low priority on attacks that are directly to the left/right. Fureya is a challenge, since her attacks are nearly impossible to dodge. Invincibility and magic attacks are your best bet. This is why choosing Fureya might not be such a good idea even though she has the best attack of the three. After he/she falls, an extremely nasty monster will appear over the body, spraying the entire battlefield with shots. Fortunately, it only takes on hit to kill.
Answer your companion's question with 'yes'. If you fail to console them in their afterlife, the game will end. Encourage them, and they will heal you to full.
Round 5: Slithe[edit]
Slithe is fast, tries to close on you, and has many attacks. Don't let him corner you against the entrance, it bring you swift deah. Move the fight out into the open area behind him. Most of his attacks only go in straight lines, so stick to the diagonals and keep hitting him. The only exception is his spikes—if he throws them, you'll have as much chance to evade them standing still as moving. This is one of the better times to resort to using a potion of invincibility.