The answer is, either a singular verb or a plural verb. It depends on what word within the phrase functions as the subject.
EX: A bunch of the boys were swimming in the lake.
Now, was it the "bunch" that was swimming or the "boys" that were swimming?
EX: This bunch of bananas is expensive.
Is it "bananas" that are expensive or just "this bunch"?
 A bunch of us are going to the movies.
Cf. The bunch (of us) is going to the movies.
Here "The bunch" functions as a substantive noun. We can omit "of us" without changing the sentence's meaning. But we can't do it here:
EX: ?A bunch is going to the movies. (odd)
The reason it's odd, "A" is an indefinite article. As a subject, the phrase needs specification, like "This bunch is going."
 An army of civil servants are demonstrating outside the legislature.
=> The civil servants are demonstrating.
Cf. An army (of civil servants) is demonstrating.
=> The army is demonstarting.
Use a singular verb if you want to place focus on the collective noun (e.g., army, bunch, etc) and use a plural noun if you want to place focus on the other noun (e.g., us, civil servants).
When in doubt, replace the phrase with a singular or plural pronoun, like this,
EX: A bunch of boys_______swimming
=> They were swimming.
=> ?It was swimming. (odd)
EX: This bunch of bananas ______ expensive.
=> ?They are expensive. (odd; "this bunch" is singular)
=> It is expensive.
EX: A sea of faces _____staring back at me.
=> It is staring back at me. (What do you think?)
=> They are staring back at me. (What do you think?)
All the best.