The plan is the secret scheme and plot against an enemy. Their plot is  the coming with elephants but Allah (swt) plotted against them:

﴾ وَيَمْكُرُونَ وَيَمْكُرُ اللَّهُ ۖ وَاللَّهُ خَيْرُ الْمَاكِرِينَ ﴿

 (But they plan, and Allah plans. And Allah is the best of planners.) (Al-Anfal: 30)

تَضْلِيلٍ (misguidance): At lose. Their plot and plan which was in secret and the other side did not see it, but Allah (swt) saw it, thus He (swt) made it into misguidance. Their plan did not lead to the purpose they came for, to that it had no value and no result.