The thing about damage output and CrE is that a thrall is a 1 headshot kill (with any weapon) and with firefly you're able to take out all his nearby buddies. So FB is very useful in CrE, it's perfect for thrall mobs.
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So you are saying...the first part only...
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Thralls are found through out and I usually pair my FB with IB and HoC so I have np with any other Hive enemy.