In terms of melee damage and defense she's about as strong as a greater vault great red wyrm. She can substitute water breath for one of her 3 melee attacks, and doesn't know any useless spells, which makes her more dangerous.
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By mid to late game you should have high enough dv/pv to easily deal with dragon turtles outside of a speedrun or quick game. The dragon is mainly difficult because of high speed plus energy cost penalty in water, and high damage from water breath. I recommend a blessed amulet of the eye, decent Pe and dragon slaying ammo. Wand of far slaying is not a good plan FYI. Plus a fully charged tp wand helps.