Although (an army) is singular form but the Arabic word indicates a plural form. Its Arabic linguistic stem word means the solid, hard and strong land. Since an army is supposed to be strong, they are named by this word.

(… جُندٌ لَّكُمْ… ) (…an army for you …): Allah (swt) is addressing the disbelievers, asking them the tell Him what force can help them.

(… أَمَّنْ هَٰذَا الَّذِي ) (Or who is it that …): This is a criticism in a form of question, meaning none of the idols can protect you, the disbelievers, from Allah’s punishment. As indicated in another verse:

أَمْ لَهُمْ آلِهَةٌ تَمْنَعُهُم مِّن دُونِنَا ۚ لَا يَسْتَطِيعُونَ نَصْرَ أَنفُسِهِمْ وَلَا هُم مِّنَّا يُصْحَبُون

(Or do they have gods to defend them other than Us? They are unable [even] to help themselves, nor can they be protected from Us.) (Al-Anbiya: 43)

The idols unable to help themselves, so how can they save you from Allah’s harm.

Hence, this is a rhetorical question directed to the disbelievers to prove that there is no power can protect them from Allah’s punishment but Him (swt). Allah (swt) uses in this verse the attribute (Most Merciful) to remind them that their existence is out of His Mercy (swt).

(  إِنِ الْكَافِرُونَ إِلَّا فِي غُرُورٍ… ) (…The disbelievers are not but in delusion): Some people may fall in delusion in this worldly life and think that they are safe from the punishment of Allah (swt) and that they will not meet Him (swt).

The Arabic word (ghurour) with the diacritical mark “u” (in delusion), differs from the word (gharour) with “a” as diacritical mark. The first one with “u” means to see something as if it is true, but indeed it is an illusion, whereas the second one with “a” refers to the one who practices this deception, that is why Satan is called (gharour) because he beautifies for the human beings the untrue things in the same way the mirage affects the humans. Satan beautifies things for human beings makes him falsely thinks that they will entertain him, but when he involves in this think he discovers that it is not real, rather an illusion. Hence, when you describe a person that he is (a deluded) it means that he is seeking something cannot be achieved. Allah (swt) says:

يَا أَيُّهَا النَّاسُ إِنَّ وَعْدَ اللَّهِ حَقٌّ ۖ فَلَا تَغُرَّنَّكُمُ الْحَيَاةُ الدُّنْيَا ۖ وَلَا يَغُرَّنَّكُم بِاللَّهِ الْغَرُورُ

(O mankind, indeed the promise of Allah is truth, so let not the worldly life delude you and be not deceived about Allah by the Deceiver.) (Fatir: 5)

The disbelievers have nothing but delusion, they are very far from the truth and following the wrong path which will not lead them to the desired target.