Then he said, God do so and more also to me, if the head of Elisha the son of Shaphat shall stand on him this day.

And the king said: May God do so and so to me, and may he add more, if the head of Eliseus the son of Saphat shall stand on him this day.

And he said, God do so, and more also to me, if the head of Elisha the son of Shaphat shall remain on him this day!

And he saith, 'Thus doth God do to me, and thus He doth add, if it remain, the head of Elisha son of Shaphat, upon him this day.'

Then he said, May God's punishment come on me if Elisha, the son of Shaphat, keeps his head on his body after this day.

And the king said, "May God do these things to me, and may he add these other things, if the head of Elisha, the son of Shaphat, will remain on him this day!"

God do so - Jehoram uses almost the very words of his wicked mother, when she sought the life of Elijah (marginal reference).
The head of Elisha - Beheading was not an ordinary Jewish punishment. The Law did not sanction it. But in Assyria, Babylonia, and generally through the East, it was the most conmon form of capital punishment. It is not quite clear why Elisha was to be punished. Perhaps Jehoram argued from his other miracles that he could give deliverance from the present peril, if he liked.

If the head of Elisha - shall stand on him - Either he attributed these calamities to the prophet, or else he thought he could remove them, and yet would not. The miserable king was driven to desperation.

Then he said, God do so and more also to me,.... He swore and made dreadful imprecations:
if the head of Elisha the son of Shaphat shall stand on him this day; imputing the sore famine to him, because he had foretold it, and did not pray for the removal of it, as he might; and perhaps had advised and encouraged the king to hold out the siege, which had brought them to this extremity, and therefore was enraged at him.

God do so, &c. - Because he had encouraged them to withstand the Syrians, by promising them help from God.