Can "titanic" well collocate with "struggle"?

Yes, certainly it can.

The Titans

Thanks.

Is it proper to say "herculean struggle" (which intends to share the same meaning with "titanic struggle")?

Yes, but it would not be so precise a use of the adjective. The entire history of the Titans, within the mythological context, was a story of struggle--of the downfall of Uranus at the hands of Khronos, of the downfall of Khronos at the hands of Zeus.

Within the mythological context, Hercules was noted for the labors he completed, in atonement for the murder of his wife and children while he was in the grip of madness. One would not be likely to see "Herculean struggle," although it would not be incorrect. One would be more likely to see "Herculean labors," or "a Herculean task."

The Labors of Hercules

Knowing about the Titans, or the labors of Hercules is basic to western culture (or it once was, i suspect children are no longer taught this mythology in schools). To fully understand the significance of adjectives like titanic, Herculean, stentorian, hectoring one has to know who the Titans were, and the story surrounding them, and to know the same about Hercules, Stentor and Hector.

I would not expect someone from outside the culture to automatically know these things. I would not expect someone from western culture to know who Guan Yu was, or to know his place in the history of the Han dynasty and the Three Kingdoms. I would not expect a westerner to know the legends and myths surrounding him. In the sense of cultural significance, Guan Yu is the Hercules of China. Of course, there really was a Guan Yu--there really was a family in Greece called Herakles (Hercules is the Roman version of the name), but i doubt if there was ever a Herakles who lived the life of the character from the legends.

Absolute Setanta bullshit. There are thousands upon thousands of words that we all understand perfectly without knowing the history/etymology of the word.