Rashi on Exodus 8:17/רש"י על שמות ח׳:י״ז

1משליח בך means I WILL INCITE AGAINST THEE. Another example is. (Deuteronomy 32:24) “I will incite (אשלח) the tooth of beasts against them”, where it signifies letting loose an animal against a person; old French inciter. English to incite.

2את הערב this means all kinds of wild beasts, serpents, and scorpions (a mixture [ערב] of noxious creatures) in a crowd (Exodus Rabbah 11:3) — these played havoc with them. There is a reason given in the Midrash in the case of each plague why this particular one and why that: God came against them with the tactics of warlike operations as carried out by kings in orderly sequence: a government (monarch) who is besieging a city first destroys its water supply, then they blow the trumpets and sound an alarm in order to terrify and dismay them — thus [their water supply was turned into blood], the frogs croaked and made a noise etc. - just as you will find it stated in the Midrash of Rabbi Tanchuma 2:3:4.