、。
That guy's a crafty old hand too, so even if you make him negotiate, he'll be a pretty tough opponent.
Literal
That-guy also [topic-(omitted)] sea-thousand-mountain-thousand is-[because-から], opponent [with-と] negotiate-[causative-させ]-even-[if-ても], considerably strong [emphasis-ぞ].
海千山千 (うみせんやません) is a four-character idiom that evokes a sea serpent that has lived a thousand years in the sea and a thousand more in the mountains — i.e. an old creature that's seen everything and can't be outmaneuvered. Used for experienced, crafty people (positive admiration, negative wariness, or neutral acknowledgment depending on context). The causative-conditional ~させても means 'even if [someone] is made to do X' — here, 'even if you let/force him to negotiate'. The sentence-final ぞ is a masculine assertive particle.