an enemy smaller than the dragon in question would be hard-pressed to survive unless it had some skill, ability or quality that worked against or around a dragon's strengths.
unarmoured fleshy creatures (say, a wolf, or a bear, any creature one might find in the woods) would be particularly vulnerable to a dragon. claws sharp enough to rend hide from bone, strength enough to shatter and crush bones (endo and exo) with a single blow, a mouth large, strong, and sharp-of-tooth enough to tear loose lethal chunks of flesh. fire hot enough to scorch skin clear to the bone. dragons occupy an irrefutable position at the very top of the food chain.
however, creatures of stone, metal, or water would be tougher to crack. stone and water are both resistant to fire, whereas metal is strong against claws. a foe made of stone might be slowly crushed and torn apart, metal melted, and water battered until it is scattered enough to loose cohesion. (though ice would shatter easy enough)
by the same token, creatures of wood would be highly vulnerable to flame attacks, one good hit with a fire breath would ignite such a being, burning it quickly to ashes.
a creature which is notably smaller than a dragon would be an easy kill, regardless of resistances, without the use of powerful technology or magic. A soldier, though he be armoured, would fall easily to an adult or ancient dragon in close-combat. what good is that armor when swatted with a claw the size of the one encased? even if the armour should not be smashed like a tin can, the shock of the blow would transmit through the body and traumatize every organ and shatter bones like glass. even if one is somehow armoured against such a blow, being completely enveloped in flame is all but garunteed to rob one of life, unless one is warded strongly against fire. and there is really nothing to be done should the warrior in question find his limbs bitten off.
a creature comparable in size to the dragon would be able to play more on it's strengths, and thus provide a more interesting fight, but unless the dragon is but a hatchling, these creatures are few in number.
a creature larger than an adult or ancient dragon would be terrifying, and those few which exist in game are indeed monstrous foes. thankfully there are obscenely few of them, being as they are all epic mobs.
given the size of the dragon, the below are what i feel should be the solo-capabilities of dragons, with modifiers given in parentheses to take into consideration the above factors (plus up to x% for faourable foe, minus up to y% for resistant foe):
- A hatchling should be able to solo any single (non-boss) enemy up to 15% (plus up to 15% against a favourable foe or minus up to 15% against a resistant foe) above-level
- An adult should be able to solo any single (non-boss) enemy up to 35% (plus up to 25% against a favourable foe or minus up to 15% against a resistant foe) above-level, and any non-boss group up to even-level
- An ancient should be able to solo any single non-epic, non-quest-boss enemy, and any group of non-boss (IE: named) enemies, in the game