。
No one can match him in strength.
Literal
Strength [in-で] him [with-に] match thing [topic-は] does-not-exist.
匹敵する ('to rival, equal, be a match for') takes に on what one is matching. The structure ~に匹敵するものはいない ('there is no one who rivals X') is a standard expression for 'none can match X.'