The above method is good but if you get killed by the squid even once in the boss battle then you'll lose all the points.
An alternative that I found by accident was:
Play Pirate Raid in King Mode. Piledrive (BX, needs to be bought) and/or Backbreak (BY, needs to be bought) everyone you can so that you can score health points from using these moves when mastered. Work your way through the level until you go into the arena with Bone Cleaver form one. Impale everyone you can, using BX and/or BY for more health if you need it.
Kill Bone Cleaver and your score should be around 21000 points :)