400 Mace Skill

[Umbranus]

The nonelite mobs in the blasted lands stopped giving weapon skill some time ago but the elite still can be used to skill weapons.
Grol the destroyer is one of the elite.

As someone else said take a fast weapon and stack as much haste as you can.

[Qtulu]

Seriously, is it random? I always thought there was some kind of invisible experience bar for each level. Wow, this is one of those revelation moments

[thoukaia]

I've always trained my weapons on the mobs in the purple area of crystalsong forest. They're level 79-80, they never end, and there arent enemy players around to interrupt me. 399-400 always took me 30-60 minutes for my DK's weapons, and always less than 10 minutes for my int-stacking holy paladin.
I dont know if it's random or if there's a hidden invisible bar, but non-white physical attacks also contribute to getting the skillup. Maybe the OP is taking 5 hours to get the last skillup because he's just auto-attacking.