。
He's a tough opponent to negotiate with.
Literal
He [topic-は] negotiating [for-させるには] strong person [copula-です].
交渉させるには ('when it comes to making someone negotiate') uses the causative + には (scoping particle). 手づよい (手強い, てごわい more commonly, 'formidable, tough') describes a difficult opponent. です is polite. The sentence characterizes someone's negotiating style — useful business vocabulary.