Personally I'd say this would be the most popular sentence among the folks who've just spotted the Vikings' presence :D
Can the singular of fiende also refer to, say, an opposing army or country in a war context, or is it just for an individual person or thing? The former is the impression I got from this sentence.
This was already answered in the comments. Arnauti says that "närmar sig" is approaches.
If you accept that "ENEMY" can be either single or plural then why isn`t "ARE COMING" acceptable?
Report it next time if you come across it again, please. It's a perfectly fine use of the collective.