Windmill is an awesome skill, at high ranks it can defeat entire dungeon rooms full of enemies in one use.
People who say it's useless to ranged or mages are idiots, it comes in very handy as a last resort protection and as a good tactic vs Mimics.
To get the skill as human, talk to Ranald at the Tir Chonaill school using the 'Skills' keyword after getting the Defence skill, he'll give you an easy quest to complete, then you'll have the skill.
To rank it, you need to pay attention to your CP relative to enemies, use the Alt key to check what level monsters are and defeat the ones that your rank of windmill tells you to.
Awful rank enemies count towards the 'defeat very powerful enemies' training requirement.
~~~
The best way to train is to form a party for yourself and set the finish rule to anyone, then you can kill enemies normally and finish every one with windmill.
Another great way to get the 'unknown' or 'defeat several strong enemies simultaneously' is to kill enemies and knock them all into a pile of 4, only one needs to be at the required level, then Windmill them all together to get loads of ranking xp.
~~~
People that tell you to go to TNN to rank are idiots too, to people with low CP, zombies are easy targets but boss level enemies don't count towards windmill training, so it's a waste of time apart from getting standard exp.
Once you have high enough CP to have zombies count as 'Awful' or 'Strong', zombies are perfect cause they move slow.
Any more questions, ask in game or post a reply below.