Your human dragonfly would need to alter the bony moorings of the existing skeleton to support wings/tails/mandibles. These are all long, strongly moving appendages which would exert a lot of leverage onto their skeletal attachment points which would need to be robust.
I will assume all materials are made of tissues already within the human repertoire.
As regards diet, diet is at essence the intake of materials needed to support energetic expenditures and maintenance/regeneration of the body (or new bodies in the case of reproduction). We do both things already with our diet. Flight might be energetically very expensive and so your human dragonfly would need more calorically dense materials. Antarctic explorers eat butter to increase caloric intake. I would expect your human dragonfly would have a penchant for eating fats and oils which are the most calorically dense materials in our diet.
I am little concerned that nothing in the human physiologic repertoire offers the hardness needed to cut through steel as the mandibles do. Teeth are the hardest thing we have and I do not think they would withstand the pressure needed. You would need some new biochemistry to allow these humans to make some hard crystalline material equal to the task. Alternatively, these are humans, and so they could augment ther biological forms with artificial addons - perhaps sapphire crowns or sheaths that harden the mandibles to steel-cutting hardness.