Instead of trial by combat, it will be a battle of wits. He will pour iocane powder into one of two glasses of wine, and let Cersei decide which to drink.The Sparrow's gotta have something up his sleeve if it's a trial by combat. He's been one step ahead of Cersei the entire time.
What Cersei does not know, is that the Sparrow poured iocane powder into both glasses, and all this time "praying", the Sparrow has secretly been building up an immunity to iocane powder.