Luke was having so much fun laughing and admiring his handiwork that Mao's ninja antics took him completely off guard as usual. It was a little ironic that, although the two boy's powers were so different, they seemed to be rather evenly matched; neither of them could gain the upper hand for very long, and there was always some point where one could bring the other down. In this case, literally. If Mao hadn't been so God damn fast Luke might've been able to reinforce his shield a little, but at best his force field only slowed Mao down a little before he crashed into Luke and, although he he was small, he was running at Luke at such a high speed that there was no way for Luke to keep his balance.
Fortunately, he's only waded in a little, since his power meant that he could splash Mao at long-distance without actually having to be close, but Luke still got pretty wet when he fell into the waterlogged sand. As soon as he was down his years of wrestling kicked in and he struggled to pin Mao, reasoning that at least then he'd be closer to the water. "So we're playing this game again," he grunted challengingly, although unlike the last time they fought, there was no contempt in his eyes. They seemed to have reached a silent understanding that this was all playful, and that's how Luke treated it.