Complex sentence: Players earn currency by killing enemy players or by destroying key enemy structures: the amount of received currency depends on how upgraded the opponent was (or which structure was destroyed) and how much of the target's total damage was dealt by the killing player.
Split into simpler sentences: Players earn currency by killing enemy players or by destroying key enemy structures. Amount of received currency depends on how upgraded the opponent was (or which structure was destroyed) and how much of the target's total damage was dealt by the killing player.