noun
natural enemy; predator; archenemy
Refers to a creature that preys on or is the natural opponent of another species. Also used figuratively for a person or thing that one is inherently weak against or incompatible with.
カマキリはバッタの天敵だ。
The praying mantis is a natural enemy of the grasshopper.
彼にとって、甘いものはダイエットの天敵だ。
For him, sweets are the archenemy of his diet.