I've been there a fair many times, so I have learned the tricks to doing it.
First, some basic info:
It has 2000 HP, which is represented by a lifebar three-to-four times the length of normal ones. Its defense is irregular, in that it not only is hard to hit but it also has some damage reduction, approximately the same as fighting a Player who uses Protection Prayers. If you do decide to Pray, use Protect from Magic.
There is some discrepancy as to if Protection Prayers work or not against it. I believe they do, because all the people I've seen get PWNED by it (two or three hit) were NOT using prayers. I know for a fact that prayers only shield you from SOME of the damage, but I think they DO help.
Now, onto gear:
People have discovered that it is vulnerable to Stab attacks, so if you are going to melee it (which I do not advise) I suggest that you use either Veracs on stab (but you risk more like that), a Leaf Bladed Sword + Rune Defender (best stab bonus), or a Zamorak Spear. Bear in mind when selecting your equipment that if you die, you have 60 seconds to loot your crap before it vanishes. You have to be perfect to make it back in time... So it's best to plan to lose anything that you drop upon death.
Note: because of the Beast's extreme HP, Ruby Bolts are by far the most effective weapon against it. However, the Beast's damage reduction ability reduces the max hit of Ruby Bolts to 100.
Note2: The beast devours any familiars that get within melee distance.
Here is what I recommend you bring if you range it:
Helmet: Neitiznot (prayer > ranged bonus... your damage comes from the specials)
Body: Black D' Hide Body (I would say proslyte body, but the accumulator)
Legs: Proslyte Legs
Weapon: Rune C'Bow
Shield: Zammy Book (A Divine or Elysian shield would be best, but no one has one except for ex-stakers with lots of third age -.-)
Gloves: Barrow Gloves
Boots:Snakeskin/None (I personally don't bring any)
Cape:Accumulator
Ring: Ring of Life/Explorer's Ring
Amulet: Fury/Glory
Quiver: 100x Ruby Bolts (your probabilty of death per trip is pretty high, so only bring 100--you will bank often enough to never worry about running out)
Inventory: 4x Saradomin Brews, 2x Super Restores, 1x Games Necklace, 21 Sharks
Here is what I recommend you bring if you melee it:
Helmet: Neitiznot
Body: Proslyte Platebody
Legs: Proslyte Legs
Weapon: Zammy Spear/LBS
Shield: Nothing/Rune Defender
Gloves: Barrow Gloves
Boots: Climbing Boots
Cape: Skill Cape (t)/nothing
Ring: Ring of Life/Explorer's Ring
Amulet: Fury/Glory
Quiver: Nothing
Inventory: 4x Saradomin Brews, 2x Super Restores, 1x Games Necklace, 1x Pure Set, 19x Sharks
Now, onto fighting the beast...
First off, it has five different attacks and attacks at whip speed. It can shoot a large ethereal fireball that looks like a blast spell, which is presumably its ranged attack. This can hit over 60 and lower random combat stats. It can also shoot a small fireball that looks like a bolt spell, that hits around 30-50 through prayer, which is its mage attack. It can melee you for up to 51 damage. It also has the attack it used in the Quest, called "The Claw." You can avoid it by simply moving, but beware because most of the KOs I've seen were when someone got hit by The Claw followed by its range attack. Finally, it can summon Dark Cores, which consistently damage anyone in a 3x3 square around them for around an average of 11 damage at dart speed. From what I've observed, each HP that the Cores drain seems to give 3x that amount to the Beast. It's definitely more than 1:1, as originally thought. These pesky minions hop around the arena and have 25 HP.
To fight the beast, everyone attacks it until it turns on someone. That person then runs under the beast until it picks a different target, similar to the "Safe Spot" strategy that was used at Zilyana. Do NOT attempt to tank this monster. YOU. WILL. DIE. I've seen at least four people be two-hit for 99 HP by its attacks. I highly, highly recommend having a team no smaller than five people, but an adequate team requires around ten people, with at least four rangers. With ten people, it takes about four minutes to kill. It takes about thirty seconds to spawn in a non-populated world.
Note: Cannons are able to be used against this boss, and are welcomed. Nevertheless, it has high resistance to them. Fortunately, it sometimes drops 2k cannonballs.
Now, as some of you know, it drops the Holy Elixir, which I guarantee will fall in price over the next couple weeks, and it also drops the highly-prized Sigils. From the amount of screen-shots I've seen over the past couple days, the Sigils are about as rare as the Hilts in GWD, and the Arcane/Spectral are more common than the Elysian/Divine. In addition, this beast drops an assortment of onyx items, 13x charms on almost kill, and a whole manner of other loot. For the complete list, go
here.
Conclusion: This boss is a hell of a thrill to fight, but in the end, chances are you will lose money. It's different than GWD, because most of the "normal" drops are quite valuable, but the risk is much greater and the cost is too. However, it is a great clan activity and the more hardcore PvE players a boss that can't be taken down by just eating/attacking.