God attacks the Pharaoh like he were some sort of demonic monster, foretelling his defeat at the hands of Nebuchadnezzar:
I am against you, Pharaoh king of Egypt, the great dragon sprawling in the midst of its channels, saying, My Nile is my own; I made it for myself. I will put hooks in your jaws, and make the fish of your channels stick to your scales. I will draw you up from your channels, with all the fish of your channels sticking to your scales. I will fling you into the wilderness, you and all the fish of your channels; you shall fall in the open field, and not be gathered and buried. To the animals of the earth and to the birds of the air I have given you as food. (29:3-4)
In The Book of Job, the Leviathan's a sort of tyrannous mega-lizard, symbolizing to the dominance of physical nature over humanity. The Pharaoh's like a little version of that same Godzilla-style monster. He represents tyranny, repression, arrogance: basically all the bad qualities a ruler could have. So God sends someone who's actually quite like him—King Nebuchadnezzar—to destroy Egypt and defeat the Pharaoh.
You've probably noticed that God's always threatening to leave people's corpses out in the open for vultures and wild animals to eat. In ancient Israel, a respectful burial was hugely important. So this image was probably pretty effective in getting across God's blazing anger.