Duke Sucellus is a two prayer fight with one mechanic that will kill you outright if you ignore it. The arena's pillars are not decoration, they are the answer to that mechanic.

Getting there

Two ways in:

Teleport nexus. Find Duke Sucellus in the boss teleport list. The nexus shows its difficulty rating, its notable drops and any requirements before you commit.
Command. ::duke takes you straight there.

The command puts you in your own private instance and does not cost an instance token. There is no reason to fight this one in a public area.

The nexus lists Duke Sucellus as Master difficulty with no entry requirements, so nothing gates you but your own gear.

Two attack styles

Duke uses both melee and magic, and each is checked against the matching protection prayer. You switch between Protect from Melee and Protect from Magic as he changes.

Praying the wrong one means taking the hit in full. Praying the right one reduces it substantially. There is no single prayer you can leave on for the whole fight.

The gaze attack

This is the mechanic that matters. You get a purple message:

Duke Sucellus turns his gaze upon you...

You have roughly three ticks before it lands. If you are not behind a pillar when it resolves, you take 55 to 90 damage.

Protection prayers do not help. Armour does not help. The only defence is line of sight.

What to do

Read the message.
Move behind the nearest pillar immediately.
Step back out once it resolves and resume attacking.

Fight near a pillar rather than in open floor, so the step is one tile rather than five. That single positioning habit is the difference between comfortable kills and dying to a message you saw too late.

Poison vents

Vents release poison across parts of the arena. Watch where the projectiles land and keep off those tiles. The vents shift, so a spot that was safe earlier will not stay safe.

Putting it together

Fight from a tile adjacent to a pillar.
Switch between Protect from Melee and Protect from Magic as he changes style.
Gaze message then step behind the pillar, wait, and step back.
Keep off the poison tiles.

What to bring

Melee or ranged, whichever you can sustain damage with while repositioning.
Prayer potions. Constant switching between two prayers burns through prayer quickly.
Food. A missed gaze is up to 90 in one hit, so you want the headroom to survive one mistake.