Remus' eyes were almost literally made for tracking movement from a distance. While it would only take closing with his opponent a little bit further to make that eyesight less effective, when harry feinted, they were still at a far enough distance Remus could track wand and hand gestures easily. Throwing up a shield to block the debris and any stray energy from the spells cast to either side, he didn't move one way or another. Rather, he let the spells fly past him and conjured a rock wall mid-air to block the first ball of energy, which shattered on impact.
Using the distraction fall debris would cause,Remus dropped to the ground and fired off a thin rope of fire to try and yank Harry off his feet as Remus rolled back to his own.