What is the significance of judging him with plague and blood?
Malbim: The beginning will be with nature, common matters; one can attribute them to chance and nature. They are normal in war ? much blood is spilled, and there is plague due to the killed. "V'Nishpateti" (Nif'al conjugation) hints that He is judged with them ? they still deny His great hand. Afterwards will be miraculous punishments - flooding rain, Avnei Elgavish, fire and sulfur. It is recognized that they are from Hashem!
What are "Avnei Elgavish"?
Rashi: They are hailstones that illuminate like gems called Gavish ? "Ramos v'Gavish" (Iyov 28:18). Radak ? we expound like two words; they are Domeh (resemble) El Gavish. Gavish is a white gem.
Rashi citing Brachos 54b: They are hailstones that began to descend on Egypt, and stood in the air "Al Gav Ish" ? due to Moshe 1 , who prayed that they not descend - "[veha'Barad u'Matar] Lo Nitach Artzah" ? it did not reach the ground.
Radak: It is a kind of hail.
54b: They descended for an Ish -- for Yehoshua, "Hishlich Aleihem Avanim Gedolos."
