Breakdown of За час до битвы он не только стоял скалой, но и смотрел орлом.
он
he / it (for a masculine noun) / it (for a neuter noun)
смотреть
to watch / to look at / make sure (in a warning)
час
hour
стоять
to stand
Questions & Answers about За час до битвы он не только стоял скалой, но и смотрел орлом.
Why are скалой and орлом in the instrumental case here without a preposition?
This is the metaphorical instrumental of comparison. It is used to compare an action to a noun, translating to "like a rock" or "like an eagle." While you could say как скала, using the bare instrumental case is a stylistic choice frequently found with verbs of motion, stance, or looking.
Can I just put any noun in the instrumental case to mean "like a [noun]"?
Generally, no. While the grammar allows it, this construction is mostly restricted to set idiomatic phrases, poetic language, or folklore. Expressions like стоять скалой (to stand like a rock, meaning to be unyielding) and смотреть орлом (to look like an eagle, meaning to look bold and confident) are established idioms. Making up new ones on the fly might sound unnatural.
Could смотрел орлом also mean "looked at an eagle"?
No, that would be смотрел на орла using на plus the accusative case. Because смотреть is used here without a preposition, and looking "by means of an eagle" doesn't make logical sense, a native speaker instantly recognizes the instrumental noun as a metaphor.
Why do we need За in За час до битвы? Doesn't час до битвы already mean "an hour before the battle"?
When specifying exactly how long before an event something happens, Russian requires the pattern за + [time amount in accusative] + до + [event in genitive]. Without за, the grammar feels incomplete to a native speaker when this phrase is acting as an adverbial time marker for the main action.