I got answers from native speakers about the following two questions. 1) What does this mean in a real situation? 2) What's the difference among "menghadapi", "menghadapkan", and "menghadap"?
The more natural translation is
"With your help, I want to confront (fight against) him."
"Dengan" in this sentence is very tricky... I also had thought that I want to face him plus you (i.e. I'm facing the South, he and you are facing the North). But "dengan" in this sentence functions as "by means of". He is my foe, and you are my ally.
This sentence cannot be interpreted as "I want him to face you" because of the verb. If you want to say so, you should use "me-kan" verb instead of "me-i" verb. "Saya menghadapkan dia" means "I made (or forced or let) him to face...". So, I as a subject do not face, but he is the action-taker. "Saya menhadapi dia" means "I confront him". I am the action-taker. For more details of the difference, please refer to a related discussion.
You might also wonder what the difference between "menghadapi" and "menghadap" is. "Me-i" verb is equal to "me-" + "kepada" etc. In this particular sentence, the nuance of "menghadapi" is "fight against". There is hostility between me and him with the suffix "-i". Meanwhile "menghadap" (without the suffix) means I and he physically face each other with no hostility.
Those explanations are based on answers from native speakers. https://hinative.com/questions/18316274
I think the original English translation "I want to face him with you" is poor and sucks! But this exercise is good for us to dive into the language.