It is a high level of Arabic wich used in ancient Arabic poetry.

The "أَفعِلْ به" form expresses wonder, surprise, admiration. A more common expression is "ما أَفعَلَه". Trying to translate the whole verses (excuse my choice of words, I'm not proficient enough for this): Oh a strike from a pious person, whose only purpose is to please God. When I recall him, I think he will be best rewarded by God. What noble people are they, whose tombs under the soil; they didn't mix their faith with injustice and aggression. (You should know this by now, but in classical poetry every verse has an independent, complete meaning)

In addition to the reason of this poem is a big story :)

This is an old kind of poetry , and this poem was written in praise of someone

I'm not sure what the first part says (it looks like فعل الأمر of أكْرَمَ), but the second part looks kind of like "They [tribes?] did not mix their religion [with?] injustice and aggression." This is just a guess; I hope that it gets you closer to an answer.