John Gill Commentary 2 Chronicles 25:8

"But if thou wilt go, do [valiantly], be strong for the battle: God will cast thee down before the enemy; for God hath power to help, and to cast down." — 2 Chronicles 25:8 (ASV)

But if you will go, do it, be strong for the battle
An ironical expression; if you will not be advised, take them with you, and try what you can do; exert all your courage, and use your military skill, and mark the issue:

God shall make you fall before the enemy ;
notwithstanding the number of your troops, and those of your auxiliaries; though some take them to be spoken seriously, and read the words, "but go you"; that is, alone, without the hired troops, and fight boldly and courageously; or otherwise "God shall make you fall"

for God has power to help and to cast down ;
to help with a few, and to cast down with many; to help without the hired Israelites, and to cast down with them.