Rashi on Exodus
ילחם לכם means He will fight on your behalf; similar is (v. 25) “For the Lord fighteth for them (להם)”; so too, (Job. 13:8) “will ye contend for God (לאל)?” and thus, too, (Genesis 24:7) “and who spoke on my behalf (לי)”, and so, too, (Judges 6:31) “Will ye plead for Baal (לבעל)?”
Or HaChaim on Exodus
ה׳ ילחם לכם, "G'd will fight on your behalf, etc." Inasmuch as the reason the Israelites had been frightened had been that they saw themselves confronting celestial forces, G'd tells them that the most powerful celestial force, He Himself, will fight on their behalf. When G'd is involved personally, even a thousand celestial forces equal to the guardian angel of Egypt are nothing to be afraid of.