Deep in the bowels of Falador park lies a beast of formidable size and laughable intelligence. You see, when Wyson the Gardener flushed his supergrowth potion into the soil to be consumed by an average mole, the mole's brain forgot to grow along with it. This means that the poor thing has yet to become aware of just how big it has become. This means that its ability to navigate its own tunnels has become severely impaired, to the point where anyone with the ability to kill things from a distance can safely slay this enormous beast. Because I'm a ranger myself this article will only focus on killing it with arrows and bolts, but the basic strategy stays the same for magic and halberds.
So why should a low level ranger try to kill the Giant Mole? Mostly just for bragging rights, as the mole's thick hide means that dealing damage to it can quickly become incredibly tedious, especially luring it into safe spots. However, whacking the mole slowly is still preferable to getting whacked by the mole quickly. Its regular drops aren't all that spectacular, but the claws and two to four hides can be sold for a reasonable amount of GP on the GE or exchanged for bird nests with Wyson the Gardener, which have the potential to yield valuable tree seeds and of course are one of the ingredients for Saradomin Brews. If the fame of slaying a beast with five times your combat level and a guaranteed profit of at least 6000 GP plus whatever it might drop (and it seems to drop 100 noted yew logs fairly frequently) are enough to motivate you to give it a try, this guide is for you.
Of course, one does not simply walk into a giant mole tunnel. The expedition will take some preparations. First of all, you will have to ask yourself if you're actually strong enough to kill that giant mole. While in theory any level 3 could do it, there is a point where it becomes all but impossible to penetrate its thick hide, and even if you do you won't damage it fast enough to overcome the rate at which it regenerates its hit points. In my experience a ranged level of at least 40 is preferable for access to green dragonhides and the Mithril crossbow. Obviously, the higher your ranged level, the better, but as an absolute minimum I suggest 40. Other than that there are no real life-or-death levels, although 40 defense might be a good idea for the use of dragonhide bodies, and 43 prayer gives you access to protection from melee, which is the Giant Mole's only mode of attack and will make it a lot easier to lure it along into safe spots. Obviously the latter isn't nearly as important as the former.
So now you've decided that you want to kill the Giant Mole, and you think you have the levels needed to do it. The next step is selecting your gear. Your equipment and inventory should be effective, yet expendable. Some food (if you don't have the 43 prayer for protect from melee), bone bolts and a Dorgeshuun crossbow* and some super-poisoned bolts*. Your equipment should be something along the lines of: coif, black dragonhide armour, broad or steel bolts, amulet of glory, rune crossbow, boots and Ava's accumulator.
(*) These items are optional (but highly recommended) time savers.
While there are plenty of 'fancier' options (Void Knight robes, gem-tipped bolts, etc.), the equipment listed here is relatively cheap and easy to obtain and gets the job done.
So now you have your inventory and equipment sorted out and you're ready to start whacking. So make your way over to Falador park, find yourself a molehil and start digging. Soon you'll find yourself all the way on the north end of the Giant Mole's lair, which hopefully will be illuminated by your lantern. The mole will spawn in the large open area right in the middle of the lair, recognizable by its summoning obelisk. However, the mole tends to wander around a bit so it could take you a while to find it. If you find other players in the lair you should just switch to another server.
Once you've found the mole, you'll want to lure it towards any of the possible safe spots. While the lair doesn't contain any of the debris that's traditionally used to hide behind, there are plenty of walls that, because of their erratic shape, are too complicated for the Giant Mole's mini-brain to navigate around. These look something like this:
As you can see, the mole's four square sized bulk will try to walk towards your one square in a straight line but can't get past the edge of the wall. These edges are most commonly found along the western wall of the main room and the eastern wall of the corridor to the west of the main room. However, they can be found throughout the lair if you know how to recognize them. Positioning yourself like this leaves you safe to peck away at its health from a distance. If you've brought them, first hit it with your Dorgeshuun crossbow special attack, followed by P++ bolts from your regular crossbow until it's poisoned. Switch to your regular bolts and start chiseling away. Once it's down to about half health the mole will start digging, resurfacing in one of eight possible locations. Once you've found it again you can lure it to another safe spot, although from now on every attack could send the mole scurrying away underground. During this phase of the fight the extra damage from P++ bolts really helps, so make sure you refresh the poison regularly. Eventually your patience will be rewarded with big bones, mole claws and mole skins, along with a random pick from the mole's drop table.