I have max UMD but I'm still not even close to being able to use UMD20 weapons at will
+ 14 ranks
+ 2 Charisma (with +2 cha ring)
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16 UMD
I can skill boost it up to 18 and then Heroism boost it up to 20 but that doesn't work in many dungeons. Yes, it works in an undead dungeon like the Tombs or Deleras but so many dungeons have a smattering of undead inexplicably dropped in the middle of non-undead areas that I find myself constantly swapping weapons.
I can grab a +4 cha item and then use skill boost when needed (easier to use Skill Boost than to have to use Skill Boost plus a potion) but it will be level 13 before I can use UMD 20 items at will.
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I'm still trying to get one myself, but you might want to run the Delara's series a few times until you get the Golden Cartouche for a +3 to your UMD. Right now, I've got armor with Command on it (for +2), but I'm planning to use the Cartouche as soon as I get one. I know, run an undead series to get something to help you with undead
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... You also might want to consider the Skill Focus: Use Magic Device feat to get an extra +3 to your UMD.
I use disruptors primarily, but I keep 2 Greater Undead Bane weapons in my bank for those quests that have named undead. I don't worry about whether the weapon is piercing/bludgeon/slashing, as I rely on the special effect damage more than the weapon damage. And of course, Wall of Fire
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