Wall beast

Wall Beasts (lvl 49) are located in Lumbridge Swamp Dungeon, and grant 105 slayer xp. A spiny helmet is a MUST for this task. To access them take not 1, but 2 sources of light (if you do not have a bullseye lantern) so that if one goes out, the other will be lit so that you will not die (you will die without a light source as to getting hit for massive damage very fast). Inside the swamp they are located at numbers 7.

http://www.go-runescape.com/img/maps/dungeonMaps/LumbridgeSwampDungeon/LumbridgeSwampDungeon_map.gif

I advise just going between the 2 southernmost spawn points, back and forth, and it should make this a rather fast task, that grants very decent slay XP.