From Quake Wiki
Shamblers are truly a force to be feared. They have an immense amount of health, and take only half damage from explosions. His pride and joy is his lightning attack, and for good reason: it never misses! Unless you can duck behind cover or retreat beyond the range of the attack, you'll be hit for considerable damage. However, the Shambler's weakness could be his proclivity to use his bare hands when he can. As long as you're within melee range, he will eschew his ranged attack for a good claw swipe or two. So if cover is not available or you're feeling particularly daring, you can run up and initiate his melee attack, then quickly step back to fire shots at him while he is swinging his arms through the air in which you stood a moment before. Then, run back forward before he begins another attack, or he will choose the lightning bolt. Repeat until one of you is dead.
monster_shambler is the entity for placing Shamblers in a level.
|targetname||The targetname of the Shambler. When triggered, the Shambler will wake up if inactive.|
|target||The targetname of the entity to be triggered when the Shambler dies.|
|killtarget||The targetname of the entity to be removed when the Shambler dies.|
The monster_shambler entity supports the following spawnflags:
|1||Ambush||The Shambler will not wake up from seeing other monsters wake up or hearing sounds.|
It also supports the standard spawnflags for difficulty and deathmatch presence.
|256||Not on Easy||The Shambler will not spawn on Easy difficulty.|
|512||Not on Normal||The Shambler will not spawn on Normal difficulty.|
|1024||Not on Hard||The Shambler will not spawn on Hard difficulty.|
|2048||Not in Deathmatch||The Shambler will not spawn in Deathmatch mode.|
This is the format used by TrenchBroom and Worldcraft. Note that the model(...) parameter is not supported by Worldcraft. This does not include the Monster base class definition, which is also required.
@PointClass base(Monster) size(-32 -32 -24, 32 32 64) model(":progs/shambler.mdl") = monster_shambler : "Shambler" 
This is the definition format used for most old Quake editors, including the original QuakeEd.
/*QUAKED monster_shambler (1 0 0) (-32 -32 -24) (32 32 64) Ambush */