Fighting the same mob all the time would soon cease to give combat discoveries, at which point you have to fight other things to begin discovering other moves.
W3 to W6 - Blockland. Special mentions: New Avalon, Tarn of Hotdog, Red October. W7 - Elvenia, til we all got lazy. W8 - Cult of R'lyeh, til the deeps called us back. W9 - Tartarus, til death/wizards came for the land itself. W10 - ???, ruined and forgotten.
You can _kinda_ fight a fox with 10 ua. You can definitely fight a boar with 30 if you know what you are doing, since the running man defnse thing is still possible.
Bats are tricky, you can't escape them once aggroed, there agi is high and you need 20 ua to soak a defense.
More to the point, high agi is far more important than high ua now from my experience. Since higher agi means more defenses/attacks means you can mess up a little more with wrong moves and have time to correct it.