[drewal]

But there you are using your adjusted THACO of -1 or 1.

In that case, you would not add 9/11 to the to hit roll, thus that roll would be 4 - hitting an AC of -5 if you have a THACO of -1 or AC of -3 if you have a THACO of 1.

Base THACO 12
Minus 4 for Strength
Minus 3 for Grand Mastery
Minus 1 for Katana
Minus 1 for Kensai as (+9) is shown on your to hit rolls - this may be -3 as shown on your player's screen, but that bonus is not translating to your to hit rolls.

Equals THACO of 3

Though that is not what appears on your player screen, that is what your to hit rolls are using - a THACO of 3.

Thus, if you roll 4 to hit, you will hit anything with an AC as low as -1 but miss everything with -2 or lower.

Thus the guard must have an AC of -2 (probably 0 or 1 for armour plus shield/dexterity bonus)

If your THACO is indeed -1, then a 4 roll would miss anything with an AC of -4 or less; in that case the guard must have base AC of 0 and -4 for dexterity/shield.