considered by the game, I've done the
following test :
Me : Level 19 Warrior
175 HP, 19 STR, 19 DEX, 19 CON, 19 INT,
8 WIS, 19 CHA (no cheats, just BIG luck

Opponent : Keldorn Level 18
19 STR (belt of xxx giant str), gauntlets
of DEX + Sunray Sword (+2)
1st TEST : AC -12
20+2 --- Touch
19+2 --- Fail
2ond TEST : AC -10
18+2 --- Touch
As you see, the game considers AC < 10.
I also did the same test with AC -18,
but keldorn couldn't touch me. That's why
i tested with AC -12 only.
A also realized (with a help of DEX editor)
a battle between me and a 19 Level
Barbarian (Minsc

My man won, but was near death - 24 HP Left.
Best regards.
Waiting for your comments.