The monster that had just landed immediately leaped again and was above my head in an instant.
Literal
Once landed monster [topic-は], instant again jumping, my head-above [to-に] existed.
いったん 'once / temporarily' marks the brief pause between actions. 瞬間 'instant' is used as a temporal adverb. ふたたび 'again / once more' is the literary variant of また. 跳躍 'leap / jump' is a formal/dramatic compound. The sentence chain — landing, leaping, being overhead — creates cinematic momentum in prose.