Saber Expert is a Level-200 boss that can only be fought after completing a puzzle. Here's a quick tutorial of how the puzzle can be solved.
The first step of the puzzle is activating all the green buttons in the two parts of the Jungle Island.
The first three green buttons are at the main, quest giver island. The other two are at the Gorilla King's island. After activating all five green tiles, go to the quest giver NPC house, right in the middle of the island above the lake.
Upon entry, you will see a hole in the floor. Go down the hole, and a message on the wall with a torch on the side. Grab the torch and go to the Desert Island.
Once you reach the Desert, look for a collapsed house with a red door. Enter through the door and you will see a curtain. Equip your torch and stand still next to the curtain. It will burn it down and open to reveal a secret passage.
Inside, you will see a wall with a phrase and a cup on the side. Grab the cup and proceed to Frozen Village.
Once you arrive at the Frozen Village, enter the cave in which you find the Ability Teacher. In there, you will find a ice stalactite with drops of water falling down to the ground. Stand underneath it and equip your cup, stand there until the cup has been filled.
Once the cup is full, exit the cave and go to top side of the village. In there, enter the house near the Sword Dealer of the East, you will find a Sick Man inside. Talk to him with the cup in hand and select the option "Help". Once you do this, he will say:
"Thank you so much! Please meet up with my son, I'm sure he will reward you."
After being thanked, go to Bobby Island.
"Thanks for helping my father,but a mob leader has stolen all my money! Please find and take him down as soon possible so I can reward you..."
Once you defeat him, go back and talk to the Rich Man. He will say:
"Thank you for getting my money back, you can keep this ancient relic as your reward."
You will now have a Relic in your item slot. Go back to the Jungle Island and prepare for a final battle. In there, find the stone building with a green tile above it. You will see a relic-shaped hole on the wall. Just equip your relic and plug it in, a door will open and Saber Expert will be inside.
He is Level 200 and is quite a hard boss if you are Level 200, too. A strategy to kill him is staying outside the door since he can't leave and bombarding him with ranged attacks. Be careful though, as he can still use his own ranged attack. After you kill him, he will drop a Saber. After completing the quest, you will be able to buy Ken Haki from Usoap if you are Level 300.
You need Level 200 to fight him or else you will get a message saying you aren't ready. In case he is not there, wait a few minutes - someone probably killed him already. He respawns every 30 minutes.
Don't worry if you die with the relic in your inventory as you can keep getting the relic from the Rich man, even if you leave the server.