Id say just about any battle in WW2 where American Marines and Japanese soldiers went face to face. Too many to name.
Yes I certainly agree that the battle in the Pacific Theatre were among one of the fiercest in history.
However, the Eastern Front was fiercest judging from the boiling hatred between two opposing sides, property damage, unspeakable Civilian death toll, toils, endurance, staggering casualty rates, and the brutality of nature.
The Eastern Front was definetly fiercest in terms of size and numbers.