。
He devised a scheme to ambush his target.
Literal
Him [object-を] lie-in-wait [one-count-一] [genitive-の] scheme [object-を] devised.
A literary/historical-drama sentence. 一計を案じる is a set literary expression meaning 'to devise/come up with a scheme,' 'to hatch a plan.' 一計 literally 'one scheme' with the one-counter as an indefinite article; 案じる is a formal verb meaning 'to think up, to devise, to worry about.' Common in historical novels and tales of clever stratagems. 待ち伏せる ('to lie in ambush for') is a compound verb: 待つ ('wait') + 伏せる ('hide, lie low') = 'hide and wait.' A precise word for tactical/hunting ambush.