As I was stating,
put on a nice suit of armor, a helmet with AC bonus and a nice ring
add your DEX bonus, and that should be it
don't sacrifice your preferred weapon style and don't sacrifice equipment you consider crucial
One common setup (listed at the bottom because of
Item spoilers) would get you to -9, which is as much as it will take. You can always be hit, but if you concentrate on doing a lot of damage that won't be that often.
I don't know where in the game you are (if ALL dragons are dead, you have not much to face anymore), but let me say:
Golems WILL hit you, Demons WILL hit you
Everything else won't do much.
Assuming there's still something left
If you're thinking about ToB however, then AC won't save you. Enemies there are though and will hit you. Again I recommend the Bard song and special abilities.
finally Item spoilers
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Armor of the Hart: -2 base
DEX bonus (18): -4
Helm of Balduran: -1
Ring of Gaxx: -2
total: -9
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you could now use the cloak of the sewers (-1) and the Ring of Earth control (-1) for additional boni, but maybe you have something better in mind to improve resistances etc.
You could also add a shield or single weapon style.
Going lower than -10 won't give you very much an edge over tough opponents, and may take away some other possibilities from you.