But the average Red Dragon has 256 hit points, and there aren’t even any weapons that do a d20’s worth of damage. At most that’s a critical hit, which would double the dmg die’s roll, in this case being 1d6 dmg, giving a muximum possible dmg of 12. At most you could do 24 dmg in one turn, assuming you are playing as a fighter and get the extra action in your turn, and you roll another nat 20 to hit.