Even other monsters fear him, so expect a clobbering. He shrugs off explosions. Good luck.
In Quake, a Shambler is a large yeti-like beast with a blood-stained shaggy coat, a very large mouth displaying enormous fangs, no visible eyes, and sharp claws. The Shambler has a complete lack of facial expression. His furious melee claw smash does enormous damage, and at a distance, he can attack by projecting a beam of electricity towards his opponent. His electric attack will only work if he sees his enemy when it ends his charging up animation. Otherwise, he'll miss.
The Shambler is a very intimidating creature, and not just for his menacing appearance. Despite having a large frame, he is surprisingly fast and difficult to outrun. His electric attack can take up to 30 health points (40 on Nightmare difficulty), and it will hit unless you're hiding behind something or are outside of his range. Because of his size, he'll try to clobber you with his melee attack if you're even a moderately short distance away; this can take up to 60 points of damage in a single attack (up to 120 in dual-hand claw). With a total of 600 health points, the Shambler is the toughest Enemy to kill in the entire game.
Since the Shambler takes half damage from explosives, avoid using the Grenade or Rocket Launcher against him. Instead, take out your Super Nailgun or your Thunderbolt and fire away. Both of these weapons will take out a Shambler in no time. The ordinary Nailgun is slightly less effective against the Shambler, as it only takes 7 health points of damage instead of the usual 9.
Taking cover a safe distance away is probably the most advisable strategy when dealing with a Shambler. If he can't see you while using his electric beam attack, it will miss. An alternative method is to get up close to the Shambler with either your Shotgun, Double-Barrelled Shotgun, Nailgun, Super Nailgun, or Thunderbolt equipped, dodge his melee attacks, and shoot at him until he dies. This is a very risky maneuver, but an efficient way of handling the creature without taking damage when done successfully. But, if you are chased by one of these, it's a good way to defend yourself. However, this strategy is only for one-on-one combat, and could cause serious problems if attempted while in the company of several other enemies. Also, avoid this technique if playing on Nightmare difficulty; his attacks are twice as fast, and getting too close for even a second will cause considerable damage, and you're likely to be killed before you kill him. The Axe also works, but you will need much more skill and luck than with the mentioned weapons. Ensure that you remain close enough to the Shambler so that he continues to use his melee attacks. It's quite hard to avoid getting hit by the Shambler's ranged attacks at a close distance.
Try not to get cornered, if this happens, your last resort to survive is blast him with the Double-Barrelled Shotgun or Super Nailgun as much as you can, hopefully he will die before you are ripped to shreds.
With other enemies, the threat posed by the Shambler becomes all the more significant. Because of how quick his electric attack animation is, you probably won't notice him charging up until it's too late. A good idea is to try and isolate the Shambler so that you can manage him without dealing with constant distractions from all around you. Depending on his position, you can either lure him away into another room and kill him first, or you can lead the other enemies off so as to take them out one at a time before turning your attention to the Shambler. As usual, grind him with your Super Nailgun or Thunderbolt, and he'll be taken down in no time. If you want to take a risk, you can pull out your Nailgun or Super Nailgun and find a place so the Shambler won't see you. Run out and shoot a few Nails, then run back in. If you keep performing this move, you'll confuse him. This strategy is only for experienced players, one misstep can be lethal.
The Shambler is relatively easily dealt with in rooms that contain a column that can be Circle Strafed around, as whenever the Shambler winds up to blast you with his electricity, the column can be used as a shield. Firing at the Shambler in moderate range around corners is also useful to a lesser extent for the same reason.
A Shambler can mow down any other monster during an in-fight, but it's hard to have his lightning beam intersect with another creature. It's actually a better approach to provoke him with enemies that have long-range attacks, such as a Vore, Death Knight, or a Scrag. While risky, causing an in-fight with a Shambler can be an incredibly useful approach when done correctly.
The Shambler is one of only three monsters (along with the Vore and Spawn's death explosion) that are capable of gibbing the Zombie (usually with his dual-hand claw attack).
The Shambler can still injure a player, even if both are on opposite sides of a wall, when using the claw or smash attack. Oftentimes, the Shambler can be tempted to continue swiping after the player has hidden behind the wall, resulting in the player taking damage even though an obstacle exists between him and the Monster.
According to John Romero, Shamblers lack eyes because the fantastic realms that they’re native to are dim, humid, and horrific, thus eyes would be unnecessary. In the same paragraph, he also confirmed that they have shaggy coats.
The Shambler also appears in RAGE, in the form of an Easter Egg. There, he appears in the secret Quake Room, in the form of a "Shambler Plush".
The Shambler's name comes from the Dimensional Shamblers of H.P. Lovecraft's Cthulhu Mythos, which are mentioned briefly in Doom's exit text.
The Shambler detecting his enemy.
The Shambler being aware of threat.
The Shambler clawing at somebody.
The Shambler's claws making contact with somebody.
↑Interview with John Romero where he states, ‘You're right about the eyes - most of the monsters don't have them. The world of Quake is dark, wet and scary and they didn't really need em. The Shambler is supposed to have a shaggy coat.’