Barrelchest, is a starter boss but does have high defense. Barrelchest is also one of the few bosses that will attack you first.
Barrelchest can be found in two places, such as Sponsor Zone, also known as Ownercape or Ownercape Zone (you must be a sponsor) and by using the Boss Teleport system.
In order to kill Barrelchest officially is to use a fast hitting weapon, such as an Assault rifle, royal sicle(u), or Ice katana . Good armor to be using is Dragonbone (U), Flame Pernix, or Vangard armor. You also need to use prayer "Soulsplit" (92 prayer recommended). Other recommendations include White Glass Wings in order to keep your prayer at max.