Perhaps then the Warhammer approach is indeed warranted and attack rolls should not even be much of a thing. Rather than stronger creatures getting progressively more difficult to even strike, let their scales become progressively more difficult to penetrate. Modifiers to attack should simply apply to damage and particularly magic armor can improve its class. Armor passively negates damage while encumbrance dictates how mobile the target is.

New attack roll: d6 - Encumbrance based

Critical Threat - rolled a 6
No Armor - 5+ to hit
Light Armor - 4+ to hit
Medium Armor - 3+ to hit
Heavy Armor - 2+ to hit
Blatant Failure - rolled a 1

Combined with some form of damage reduction that thickens Dragon scales, even on a per round basis such as striking the weak points you've already cracked, combat becomes more about being able to damage the enemy at all rather than a game of Dodgeball.