Def is much more important than hp on her since she'll definitely be getting doubled, and refined lightning breath helps make up for the lost hp, so I would personally go with -hp +def
However, if you pull a -Spd/+Def A!Tiki, then I suggest changing your merges to her. Like the first comment said, most dragons have a standard quick riposte build, which works well with their bad speed and great defenses.