One wins a game, obviously, by scoring more points/goals than the other person or team - that is, by outscoring them.
"Jameson won the title BY outscoring Jeffries on a unanimous decision."
In colloquial speech, with omit the 'by' as being obvious, understood as being there, and the sentence would be (note the use of comma)
"Jameson won the title, outscoring Jeffries on a unanimous decision.
(You have used 'on' - this must be boxing. Otherwise, we would say, 'in a unanimous decision'.)