RuneScape players can earn up to 40k experience weekly through killing the Ashdale boss. Players are allowed to take on the boss twice every week, gaining as a reward the choice between just over 20k gold and a black pearl. The black pearl rewards experience much like a prismatic star, giving up to just over 20k experience if level 99.
Bring food and weapons of choice and wear good armor. Home-teleport to Ashdale. From the lodestone, go across the bridge and run through a pasture west, then hook around the path north and enter the cave. Right clicking on the chest there allows banking. Left clicking allows the RuneScape player a rematch with the boss.
Kill the tentacles, one at a time. Max hit is 40, so it takes about five hits to kill them. Tank the first one, incurring damage from it hitting the boat. Using ranged, stand in the same spot and pick off the rest. Once one tentacle is dead you can safe the others. I only brought food rather than trying to find a safespot.
When the tentacles are dead, run forwards and attack the boss. Praying soul split and using a Saradomin godsword negates the normal damage, but food is needed if you are caught by the explosions. As soon as the boss becomes immune, see if there are marks on the floor and run if they are under you. Failure results in a hit of around 1k. Keep health above 2k to be safe. A good weapon and full action bar makes quick work of the boss, as long as you avoid his specials.
After the fight, left click on the bank chest to claim the black pearl. These cannot be banked, but are consumed to grant bonus experience. Select the skill that you want the bonus experience in, then train to receive the double experience or allow it to be collected through passive gameplay. You can fight the boss a second time after restocking, in order to save another trip back to ashdale.