James had been in quite a few duels since going on the run, but this was unique. For one thing, they were in the Ministry of Magic. For another, his opponent was invisible, and that made him or her difficult to predict. From behind his own column, he sent a series of spells in various directions across the room, criss-crossing with Kingsley's, intending to cover the whole area where the attack could be coming from.
When the flames began, he did indeed get the hint. Interesting, though, that this person seemed much more interested in stopping them getting inside the Ministry than keeping them there. At least, that was certainly how it seemed. Whether they continued farther into the Ministry to find Remus or not, they would have to deal with their current predicament first, so he wasn't terribly worried.
Their opponent was moving, and so would he. Moving fast, he got behind another column, ducking a spell that came his way as soon as he moved, and aiming one back moments later.