"凭他们本身的实力" can be translated into English as "based on their own capabilities/strengths" or "through their own inherent abilities".
Here are some example sentences for context:
They succeeded based on their own capabilities. （他们凭自己本身的实力取得了成功。）
The team won the game through their own inherent abilities. （这支球队凭自己本身的实力赢得了比赛。）