"Lay thine hand upon him, remember the battle, do no more." (Job 41:8)
Two things: This is God speaking, one. About the devil. Rewind a little to both the opening of Job's book and also the first part of chapter forty-one (stay with me here). It was Satan who initiated the attack on the entirety of Job's life (see Job 1:9-12) and also he to whom God is referring when He talks about "Leviathan". It's almost like a higher form of reference, akin to an argot only God has and knows how to use correctly: the symbolism inherent to the natural world. Read through the whole of Job 41 if you got the guts.
"There go the ships; there is that Leviathan, whom Thou hast made to play therein." (Psalm 104:26, emphasis mine)
When walking through the sea, keep in mind there's a monster about. The word "leviathan" (sounds about the same in English as it does in Hebrew) connotes a sort-of twisted and inescapable auspice, the embodiment of which fits the "sea serpent" perfectly. A milder and revisionist definition might be "whale". If you think a great white shark is the worst thing out in the deep, extend your imagination back to the megalodon and maybe you get some idea the seriousness of the implication. God asks Satan early on in Job (1:7), "Whence comest thou?", the devil answers honestly: "From going to and fro in the earth, and from walking up and down in it." The same idea is echoed in Peter's first letter (5:8) saying he "walketh about, seeking whom he may devour." Granted, Peter metaphorically refers to the devil as a "roaring lion". Perhaps a, uh, sea lion? Hold on a second, we need to stay serious. Did God just tell Job to touch Satan? Maybe we should rewind a bit more.
"And I" this is God speaking to Satan. "will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel." (Genesis 3:15)
After the Fall, it would seem Eve and her daughters were given a special gift of hatred for Satan and his...offspring. Granted, there's the "serpent" and there's the "sea serpent", or "leviathan". But metaphorically either one is referring to the devil himself. And if you've ever dealt with the devil, i.e. Satan, you know he's a force to be reckoned with. At this stage in my life, I see Lucifer as a litmus test for what God's kids are capable of overcoming (see Revelation 12:11). Take that with a grain of salt but know also that we must overcome ourselves as well. All with God's help, of course.
"In that day the Lord with His sore and great and strong sword shall punish leviathan the piercing serpent, even leviathan that crooked serpent; and he shall slay the dragon that is in the sea." (Isaiah 27:1)
Before we go any further, I find it interesting to reflect on the sea of titles in your Christian book section. While some are directly aimed at defeating the devil for your life (and that call him out by name in the title), others extol and explain God in ways unique to the author and that the author thinks might make sense to you. Pretty obvious, right? The ratio of books glorifying God and that don't even touch on the devil one iota is probably ten-to-one. This makes sense as God is Creator and we should be living toward Him and then only when He reveals what's going on in the shadows (Shallows? Deeps?) should we really focus in on the devil. One book I saw led with a brief footnote on how—temporarily abandoning proper English—they elected to not even capitalize the devil's name throughout the text, so undeserving is he of any consideration. Okay. But think though about what God does for Job. Chapter forty-one details by way of analogy—the aforementioned "argot"—the splendor of the sea serpent. When you think about it, all the arrows in Lucifer's arsenal were forged using gifts that were originally intended to remain holy and righteous. It's almost as if, God uses "Leviathan" to point out his handiwork and implies that it deserves a sober respect. He says "I will not conceal his parts, nor his power, nor his comely proportion." (verse 12) Not one mention, though, can really be seen as a truly admirable quality and when God sows up the lesson/chapter with "he is a king over all the children of pride", we know that we're not playing around. There's a lot of pride, to be sure, in the sea of humanity. But in Genesis when God tells Satan that one of Eve's children "shall bruise [his] head", it's referring to Jesus and his victory ensured by His atoning death and resurrection. That same victory does He give to us. Pray, praise, persevere. Press on through. The victory belongs to you in Jesus Christ.
"And God of peace shall bruise Satan under your feet shortly. The grace of our Lord Jesus Christ be with you. Amen." (Romans 16:20)
"Thou brakest the heads of leviathan in pieces, and gavest, him to be meat to the people inhabiting the wilderness." (Psalm 74:14)