Again, Sal you are completely off the mark.
 
First, you seemed to be confused as to which "against" phrase is under discussion. In the first part of the verse "Egypt went up 
against the king of Assyria" but they were NOT in opposition to each other, nor did they kill each other then; the Egyptians and the Assyrians were 
allies here. This first "against" is in the phrase that could be misunderstood as approximately meaning 'hostile opposition or resistance'. The "against" in the second part of the verse took place between Egypt and Judah (Josiah was essentially aiding the Babylonians). Necho didn't really intend to fight with Josiah at that time (see 2 Chronicles 35:20-24).
 
Second, you did not appear to fully appreciate robycop3's reason for "against" being acceptable 17th century English. He contends that the word is being used in the 'preparation for or in anticipation' sense [see definition 8 below]. Roby offerred Genesis 43:25 as an example of this useage. Your response does NOT indicate "against" in that sense. (For example, substituting 'prepare for' in place of "be against" in your response above doesn't result in your whole statement making too much sense).
 
FYI, the first "against" in 2 Kings 23:39 is NOT even supported directly by an underlying Hebrew word. The phrase "against the king" is based solely upon a form of the Hebrew word 
melek (Strong's #4428) a masculine 
noun meaning 'king'. Therefore the word "against" is actually being provided by the translators. However, the in Greek LXX has translated 
epi (Strong's #1909) a 
preposition here (meaning 'upon, on, at, by, before, over, against'. In the KJV NT this Greek word is usually translated as "on" 196 times, "in" 120 times, "upon" 159 times, "unto" and "to" 41 times each.
 
Now, the second "against" IS supported in the Hebrew by 
qir'ah (Strong's #7125) a masculine noun, meaning 'to encounter, befall, meet' (in the AV translated variously as "meet" 76 times, "against" 40 times, "come" twice, and "help", "seek", "way" once each).
 
The English word "against" in Genesis 43:25 is supported by a different Hebrew word 
`ad (Strong's #5704) which is a preposition, meaning 'as far as, even to, until, up to, while, as far as' (in relation of space, of time, or of degree).
against 
prep.
1. In a direction or course opposite to: row against the current.
2. So as to come into forcible contact with: waves dashing against the shore.
3. In contact with so as to rest or press on: leaned against the tree.
4. In hostile opposition or resistance to: struggle against fate.
5. Contrary to; opposed to: against my better judgment.
6. In competition with: race against the record holder.
7. In contrast or comparison with the setting or background of: dark colors against a fair skin.
8. In preparation for; in anticipation of: food stored against winter.
9. As a defense or safeguard from: protection against the cold.
10. To the account or debt of: drew a check against my bank balance.
11. Directly opposite to; facing.