Any weapon that has Banishing will definitely help. AS David mentioned, the adamantite longsword is probably your best bet, unless you like to use Two handed weapons, then get an adamantite greatsword. You will need the help of all the people there especially Sir Valen. Although the peasceful resolution will help you avoid fighting the beast altogether. Anything that does elemental damage doesn't really help because it has a decent resistance to them. Collect as many healing potions as you can especially the ones you can get from Ashen minotaurs and the med kits from the ark. I find that potions of restoration are only useful if you're a cleric or wizard. Go around the forum and try to find the walkthroughs for more help.