The First Data-First Financial merger agreement may have shaken the payment systems landscape, but it was all in a day's work for John C. Elliott. Or maybe 14 years' work, to be more precise. But in the normal course of events, just the same. Mr. Elliott spent much of his career building transaction processing businesses, often by piling one acquisition on top of another to create the critical economies of scale. The $6.7 billion deal announced Tuesday was in many ways the ultimate realization and culmination of his vision. Mr. Elliott, with the title of strategic adviser, has been a key management team member at First Data Corp. since March when it acquired his former company, Card Establishment Services Inc. of Melville, N.Y. With that $500 million-plus acquisition, First Data overnight became the No. 3 processor of credit card transactions for merchants.