Complex sentence: Optimus engages Megatron, Starscream and Grindor into a vicious fight in a nearby forest; he is losing badly at first, but eventually overpowers Megatron, defeats Starscream and kills Grindor.
Split into simpler sentences: Optimus engages Megatron, Starscream and Grindor into a vicious fight in a nearby forest where after losing badly at first. Optimus overpowers Megatron, defeats Starscream and kills Grindor.