Description
Even for a melee combat master, escaping the attacks of multiple opponents is not an easy task. When dealing with one opponent, you cannot avoid the attacks of another one that takes advantage of the situation. This skill enables you to escape from danger by exchanging a little of your life. Knock out the surrounding enemies attacking you and escape. This skill is even more useful when you've been knocked down by successive attacks of your opponents.

Details
Knocks down all opponents within the radius, unless they are defending.
Windmill ignores the target's Protection when calculating critical rate.
Windmill will take 10% of the player's Life upon being unleashed, even when the player is in deadly status.
If the player's maximum Life is 10 HP or under, each Windmill will take 2 HP instead of 10%.
Windmill's radius, in general, increases with model size. A Golem will have a gigantic windmill radius compared to a Human character. Along these same lines, a Giant character will have a larger windmill radius than a Human of equivalent skill rank.
Notice that there is a difference between Windmill's range (the boost % given by the skill) and Windmill's radius (the base hit distance for wm).
If the player has been knocked down or back by a previous attack, he/she will be invincible for a very brief moment just right before the skill is loaded – if one attempts to attack the player a few milliseconds before before Windmill is loaded, the skill will cancel itself, and one will completely "miss" the player, showing attack animation without damage and stun. This interrupts the opponent's chain of normal attack and gives the player a short chance to retaliate or run away. However a combo attack may be applied after the skill cancel.
The player will also be invincible for a brief moment during the spinning animation phase.
However, the Self-Destruct explosions caused by Lost Sahagins, Skeleton Squads, and Bomb Steeds will violate this principle.
Escape Bombs or Fireball will not hit the player. A Self-Destruct attack is the only explosion that can harm the player during the spinning animation.
It is possible to avoid the first few thunder bolts of high ranked Thunder if one can execute Windmill right after the lightning bolt (Mana Deflector has to be activated).
If Ice Spear activates Mana Deflector and the user uses Windmill, the explosion effect will interrupt the Windmill invincibility animation.
The spinning animation lasts for about 2 seconds (possible deviation 0.2 seconds).
As of G7, only bare-hand damage is calculated when Windmill is used with a bow equipped.
Windmill does not inflict any wounds.
If a monster using defense is hit by windmill, it may run up next to the player but not attack. Though this seems like the perfect opportunity for the player to attack, the player must NOT do so or the opponent will strike back. They should simply load and unleash Windmill again. This does not apply to all monsters (for example, a Succubus).
Even if a monster doesn't get hit by Windmill, it still pauses during the spinning animation. The player will get hit if he/she tries to attack after.
If a player uses basic attack, he/she might get lucky and not take damage.
Some monsters, such as a Bugbear, will sometimes attack you before you can load another Windmill.
If you are being aggroed and something reaches you right after the animation, it may attack before you start loading Windmill again.

Players can execute a counterattack with Windmill by preparing the skill while being knocked down (usually against a wall) by an opponent. As soon as the skill is finished loading, it will automatically execute as long as a target is within range. This is how to fulfill the "counter with windmill" requirement in earlier ranks. An easy way to know when to do it is when the enemy's combo is shortened because it is attacking you frequently. One of the most recommended methods is to have a spider pin you against a wall and then load the skill when it attacks only once instead of three times, causing Windmill to automatically activate if done correctly. Auto-combat is said to help, but is unconfirmed.
There is a strategy known as an "auto-mill." It is executed in mostly the same manner as regular windmill, except the player must press the mouse button before loading the skill (before the bubble appears, not before it clicks) and continue to hold it down while the skill is active. (If done correctly, the game will flash the "no target in range" message repeatedly.) The skill will automatically execute if any monster is within the Windmill radius or when a monster comes within the radius. If a monster has been targeted, Windmill will not activate until that particular monster has come within range.
This is, however, very susceptible to lag.
Many skills can be loaded while the player is still in the spinning animation phase of Windmill.
These include Smash, Defense, Ranged Attack, Final Hit, Magnum Shot, Arrow Revolver, Elven Magic Missile, Mirage Missile, Support Shot, Healing, Magic Shield, First Aid, Thunder, Charge, Playing Instrument, Stone Throwing, Giant Full Swing, and Life Drain.

Obtaining the Skill
Humans: Talk to Ranald about Skills after you have learned Defense. He will then give you the "Ranald's Armor Delivery" Quest. Upon completion, the player will receive the Windmill skill.
Elves and Giants: Finish the beginner quest "Windmill, the Crown Jewel of Melee Combat."
After, prepare 3 AP (Unless character is an elf) or it will not be effective