Put all points into your weapon skill of choice at the start.
Travel around and look for a groundhog/buzzard.
Remove (P) your weapon and leave your shield on.
(C) Change combat preferences, stand ground on the bottom two.
Hold . If it doesn't repeat, go to options, controls/keys, look for adventurer controls "Wait" and turn it to instantaneous repeat.
Bluetac your . key or something, find a way to have it repeat without you holding down the button, you can probably use a program but I use a bottle cap and a watch on top of that.
Go read a book, watch TV, exercise or make food, when you come back in 10-30 Minutes you should have legendary, perhaps multiple times over, as I understand getting Legendary 11+ Is better than Legendary 5+ But the exact effects of a higher legendary combat skill evade me right now.
It's best to do this when you have at least a helmet on, you could do it when you have a full steel set on you, that way you train your armor user skill as well, but from my knowledge armour user increases your speed the higher it is, reducing encumbrance effects (very important imo, as being able to attack .5 More times than pretty much anything is a huge advantage when you take into consideration opportunity).
This is cheesy and broken, you will have a superman-like character if you leave it there long enough and be able to punch out Cthulhu.
Weapon skills are harder to train or rather take much more time than any defence skill because of this way of training them, you can reach legendary+20 On all your defence skills twice before you could reach legendary from nothing on any weapon skill (except hammer skills because flesh balls are an option), so it's easier to pump one up to grand master and go from there.
Also this happens to massively increase your fighter skill too which is always good as I believe it increases your opportunities? Not sure what, but I do know that someone with master fighter will consistently beat an otherwise equal opponent in arena.