The word "striding" seems like a weird choice of words to describe something a bird does. Birds fly, they don't stride, or take long steps (unless we're talking about Big Bird from Sesame Street or an ostrich, but you know what we mean). We usually reserve the word stride to describe the steps of someone who really knows what he or she is doing. As in, "She strides masterfully across the room and seizes the crown for herself." So the choice of the word striding here might suggest the bird's extreme skill and know-how.
[…] the hurl and gliding Rebuffed the big wind. […] (6-7)
This bird's so tough that he's able to "rebuff," or to beat back, the powerful wind so that he can hover in place in the air.
[…] the achieve of, the mastery of the thing! (8)
The speaker is totally blown away by the bird's skill. So blown away, in fact, that he uses the verb "achieve" instead of the noun, "achievement," that we'd expect in this sentence. Maybe he's trying to emphasize the bird's action and motion by using the verb, or maybe he's trying to make us do a double-take, and to think about familiar words (like "achieve") and familiar sights (like birds flying) in new ways. Or maybe it's a bit of both. Hopkins had mad skills, after all.