In Immortals, the Titans battle the gods (i.e. Zeus and buddies). In the film it's shown that the gods have extreme physical capabilities: mainly speed and strength. When they battle the Titans though, it seems like the Titans are faster than humans, but nowhere near as fast as the gods. As a result, the battle between them is practically a slaughter. It seems to me that the Titans should have been at least equal to the gods, if not stronger. Why were they so weak?
It boils down to poetic license (why don't they adhere to Grecian battle tactics and ancient Greek architecture?), but I am not sure I agree with your analysis. The way it looks, at least to me, is that the difference between the two sides is that the gods seem to be better warriors. In comparing them, I am reminded of Bruce Lee vs. your ordinary warrior (I once saw a film of Bruce Lee knocking a man halfway across the room without moving more than half a foot...). Imagine what a battalion of Bruce Lee's might do to a legion of regular warriors, even if the warriors were more well-trained than most.
As a side note, there are a number of different stories of the overthrow of an older generation of gods by a a younger, and in the case of the Greeks, that tale is largely lost.
My general experience with Grecian myth, however, is that the younger generation is considered less barbaric and more civilized (Titans largely being portrayed as cruel simpletons), which may be one of those cross-cultural undertones of generation warfare combined with the "natural man vs. civilized man" (cf. Epic of Gilgamesh). This would be similar to the Norse pantheon and their war with the giants. We, as a culture, have a tendency to have the view of "smarter = stronger = better" and give all of the good attributes to the heroes. This is a bi-product of our Greco-Roman heritage.
As another side note: Bruce Lee is the only one to ever beat Chuck Norris.
This is addressed in the back-story provided in the opening voiceover. In Immortals, the 'Titans' aren't the progenitors of the Gods, they're simply another faction of immortal beings who originally dwelled in the 'Heavens' alongside each other.
These two sets of immortal beings fell out (for reasons not given, but clearly fashion-related), and a vast war ensued. One side was evidently more numerous while the other side seems to have been more powerful individually. On top of that, one side was evidently more skilled in forging magical weapons which one assumes would add a +10d20 to every attack.
VO: There was a war in the Heavens. Immortals, once thought incapable of death, discovered they had the power to kill one another.
Lost in this war, was a weapon of unimaginable power: The Epirus Bow.
The victors declared themselves Gods. While the vanquished were renamed Titans
After the battle, the remaining 'Titans' (those that surrendered or were captured) were imprisoned, while the Gods continued to dwell in Heaven and wear ever more silly hats and outfits.
Once released, much the same occurs as before. The more numerous titans attack the more powerful gods and while they may face individual defeat their sheer numbers wear down the god's defences.
Deities and Worship
The simplest answer is though the Titans, who were descended from the primordial beings of Uranus and Gaia may have given birth to the later generation of gods, the ascendancy of Zeus and his brothers/court, as well as their offspring directed the energy of worship from the earlier Titans toward themselves. During a great war between the Titans and the Olympian deities called the Titanomachy Zeus imprisons the Titans in Tartarus after the battle. This battle may have reduced the efficiency of some of the Titans since after their loss and replacement, they would no longer be gaining fresh spiritual energy in the form of worship from humanity.
There was a common theme of father against son among the Greek deities where Chronos rebelled against Uranus, and later Zeus against Chronos. Zeus would later fear the same from his own offspring, so he first imprisoned the Titans so they would no longer be a threat. Then for a time, he would even attempt to prevent a later generation of Gods from usurping his power by consuming any potential offspring. This did not stop Athena from eventually bursting free from his head, fully dressed for War. However, his libidinous nature ensured there were plenty of demigods out there, performing heroic deeds on a regular basis.
The reason they lost in the movie is because Immortals is based upon the Greek myths. In the Greek myths, the Gods beat the Titans.
Now why did the Gods beat the Titans in Greek mythology? The most straight forward is that the Titans were never worshiped. When man invented the myths, they would want their Gods to be the winners. It makes no sense to worship the losers of a great battle.
First of all I watched the movie the Immortals and failed to live up to the mythology. The truth is that the Titans are more powerful then the gods. The only reason the Titans lost to the gods was because Zeus promised many of the exiled and imprisoned giants freedom if they would fight on the side of the gods. You see Krunos the Titan would exile many of there offspring because they were born ugly or dis-formed. Before the battle of the Titans began the offspring of the Titan were promised freedom by Zeus if they would fight for him. There were the One hundred handers, the cyclopes and, other giants. So when the battle began between the Titans and the gods, the Titans found themselves out numbered. Plus there were Titans who switched alliance and fought on the side of the gods. The gods with all this extra help eventually triumphed over the Titans (the old gods) .