However, 74 damage from a mundane sling? No. That's absurd. I'm a reasonable man, willing to consider lots of different ideas, but a mundane sling doesn't do 74 points of damage no matter who wields it. David can kill Goliath with a mundane sling, because he is favored by God and also because he's not playing D&D.
To fix the dragon, I propose a change to saving throws. Back to 4e/SW SAGA. Have that dragon attack Fort, or at least saving throws other than Reflex.
When Star Wars SAGA edition first came out, I didn't like its idea of having saving throws as defenses. It wasn't until running 4e that I saw why this was a good idea.
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For that dragon, I would make his tail attack be an attack versus Fort. The tail is so big it's hard to dodge, but you only take damage if it sends you flying. Therefore, it's an attack versus Fort that also pushes you. Perhaps this is doable without changing the saving throw system, as if the tail were a breath weapon that allowed a Strength rather than Dexterity saving throw.
To fix the dragon, I propose a change to saving throws. Back to 4e/SW SAGA. Have that dragon attack Fort, or at least saving throws other than Reflex.
Chalk me up as a fan of Fort/Ref/Will, each informed by one of two abilities (Str/Con, Dex/Int, Wis/Cha). I know the desire to return to "roll a saving throw" is great, but sometimes you have to cut out your favorite part of something for the rest to "click" into place.
IMHO, the rogue should be doing something like 10% of the dragon's total hp per round, on average. On a successful Sneak Attack, the rogue should be taking something like 25% of the total (an assassin rogue, with an optimized attack that can't be repeated in a single fight, should be able to take out sbout 66% of a dragon's total hp). Conversely, the dragon should be taking out 25% of the rogue's hp with a single attack, or 50% with a seldomly-used breath weapon.
Thinking of these figures as percentages would then guides through the actual damage calculation.