The most armoured monster in the game is the Gold Dragon. But my magical tables show that 30% chance to hit a gold dragon corresponds to +17 to hit. For other monsters, the sufficient bonus is much smaller.Your attack bonus should be at least +15 (Hades) + 12 (Excalibur) + 12 (minimum of 12 levels in Sword) = minimum of +39, which would give 44% to hit against the gold dragon.
So — I can't believe 30%. Either you have been extremely unlucky, or it's just your subjective feeling.
The base unmodified damage for this combination has a minimum of 3d4 + 3d3 + 15 + 12 + 12 = 6~21 + 39 = 45 ~ 60. If you are dealing 40 to 50, it would mean either a highly resistant opponent, or a level 1 knight who has nothing more than those two swords on him (and 12 in Sword).