Is the main battle of the enemy for the mind?

Saying that the 'main battle' is for the mind would possibly be an oversimplification. Better to say the main battle is over 'faith' and the mind is one of the primary faculties which influences our heart in that matter.

The fact is once we are deceived about the truth in our mind it is very difficult to have faith. Why else would the preaching of the gospel be so central to our faith, if the mind was not important. So, for the first part of your question I would say, yes, in some ways it is primarily the mind. However, having a weak mind does not equate to weaker or stronger faith at all! I have met many people who not only had weak minds but poor theology and who yet under the poor light of it seemed more faithful to God then I was at the time. This means there is much more going on. One person may be the most correct theologian and teacher and yet live in much wickedness and another may be quite confused theologically, and be very genuine and sincere in their faith. How can this be? Who really knows yet it can be!

Faith transcends mind and emotions and passes beyond the veil of our understanding. People with mental disease and mental weaknesses can cry out to God and be saved. Also the Holy Spirit is able to awaken the mind in a subconscious level, make one sensible of their sin and show hope in the salvation offered in Christ, in ways we can't comprehend. I am sure many mentally ill people who have took a hold of Christ by faith and through the power of the Holy Spirit, will be surprised to find great rewards awaiting for them in heaven, while some esteemed biblical scholars will arrive in heaven as one escaping through the flames, with nothing but their own soul in tact. Of course in both cases the joys of heaven will be glorious, I only mean to say that those things which are despised are often valued by God and he will expose the hidden intentions of our heart when we judges all things as they should be judged. God takes into account our situation and rewards accordingly.

If I was mentally ill I would take comfort that Jesus was my wisdom. Not my mental or emotional capacity:

But God chose the foolish things of the world to shame the wise; God chose the weak things of the world to shame the strong. God chose the lowly things of this world and the despised things—and the things that are not —to nullify the things that are, so that no one may boast before him. It is because of him that you are in Christ Jesus, who has become for us wisdom from God—that is, our righteousness, holiness and redemption. Therefore, as it is written:“Let the one who boasts boast in the Lord.” (1 Corinthians 1:27-31, NIV)

God's grace is sufficient to overcome any weakness. All weakness expose us to more temptation and sin and where sin abounds grace much more abounds (Rom 5:20). That grace is sufficient to overcome anyone's mental illness is undeniable. We see the logic of the gospel when Paul himself wanted some weakness to be removed:

Three times I pleaded with the Lord to take it away from me. But he said to me, “My grace is sufficient for you, for my power is made perfect in weakness.” Therefore I will boast all the more gladly about my weaknesses, so that Christ’s power may rest on me. That is why, for Christ’s sake, I delight in weaknesses, in insults, in hardships, in persecutions, in difficulties. For when I am weak, then I am strong. (2 Corinthians 12:8-10, NIV)

Now if God's power can be made perfect in weakness and His grace can abound much more where sin abounds and since he tends to cherishes what the world despises, is it not very Godlike to glorify his name in the soul of a man who has mental deficiencies, while bringing low those who boast about their theological knowledge? You bet he will. He is God and will not share his glory with anyone.