Mentioned in various spots in the Bible, the Leviathan is described as a colossal serpentine creature. It is very heavily armored, able to thwart most types of melee weapons, along with some low-velocity projectiles. In addition, it is capable of breathing fire (or acid, depending upon which translation you use), allowing it to pretty badly mess up any human force which dares attack it. As such, it is best described as a "bioweapon"; whenever God wants to mess up some particularly obnoxious unbelievers, He sics the Leviathan on them.
Since the Bible et al don't really describe the Leviathan in greater depth than I have already described, my question for you is this: Given these features, what would a God-crafted Leviathan be like physically? A good example of the type of answer I am looking for is @IndigoFenix's answer. It does a pretty good job, but I am still interested to in what the rest of you think.
"Evolutionary pressures" are not a concern; This is the Judeo-Christian God we are talking about; He is omnipotent, and thus doesn't need to mess about with evolution when creating His ultimate bioweapon.
Nota Bene: I realize that there is a similar question out there; however, this is different. That question assumes evolution as a cause, this question assumes that Jehovah created it.
A Final Nota Bene: I am looking for a creature that is biologically feasible. This means that, although supernatural means were involved in its creation, it cannot depend on magic for its continued existence.