These spawn behind the mythril door, in the same area as the Treasure Coffers. You can camp just inside the Mythril Door and fight there - the leeches are non-aggressive. Be careful if the weather turns Thundery tho, as there is a spawn point a little way in front of the door and the elemental can wander close enough to detect you.
These are not WARs - they are MNKs. They hit twice/round, once with each claw/flipper/whatever, and they Counter. These will strip Blink/Utsusemi/Aerial Armour really fast, but are fairly simple to straight tank as they don't hit terribly hard.