Contradiction with Exodus 14:14
Joshua 8:14 involves a battle and the use of strategy, whereas Exodus 14:14 emphasizes that the Lord will fight for you without needing human strategy.
Exodus 14:14: The LORD shall fight for you, and ye shall hold your peace.
Contradiction with Matthew 5:44
Joshua 8:14 involves an ambush against enemies, whereas Matthew 5:44 teaches to love and pray for your enemies instead.
Matthew 5:44: But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
Contradiction with Romans 12:19
Joshua 8:14 describes taking vengeance by ambushing, whereas Romans 12:19 instructs not to take revenge, leaving it to God's wrath.
Romans 12:19: Dearly beloved, avenge not yourselves, but [rather] give place unto wrath: for it is written, Vengeance [is] mine; I will repay, saith the Lord.
Contradiction with Isaiah 2:4
Joshua 8:14 involves going to battle, whereas Isaiah 2:4 envisions a time when nations will not learn war anymore.
Isaiah 2:4: And he shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruninghooks: nation shall not lift up sword against nation, neither shall they learn war any more. [pruninghooks: or, scythes]
Contradiction with Proverbs 20:22
Joshua 8:14 involves retaliation and conflict, whereas Proverbs 20:22 advises not to say "I'll pay you back" but to wait for the Lord's deliverance.
Proverbs 20:22: Say not thou, I will recompense evil; [but] wait on the LORD, and he shall save thee.