As his opponent suddenly begins to dart away to her side, Roy realizes that she was immune to her own rain. At first glance, the black colour made him think it was oil, however, no opponent would be stupid enough to get caught in their own trap, allowing him to assume it was something else, and the Earthen structure had stopped the black rain by mere chance.
His opponent however continued to run, not realizing Roy had just noticed it's not rain. As she made the step and focused her Fire chakra into the ground below him, Roy suddenly sensed the chakra from the area below him, at first he had assumed it was some form of an Earth jutsu but the heat could easily be differentiated from that of a normally created Earth jutsu. As a response, before the pillars could fully emerge, Roy would jump up, summoning a wall of Water to the ground below him as he did so. The 'Wall' of water is summoned from the Water source, and would be made to be quite dense, easily extinguishing all 3 pillars of Fire as they come out, whilst the water remains, although now at a weakened rank of B, though that's of no matter as it's dropped the moment your jutsu is countered.
At this point, steam would have formed and Roy could sense you sending Lightning chakra high up into the sky, shooting them off. At which point he'd have already have merged into the ground and began travelling in an arc to his left, getting out of way of the Lightning pillars as the whole Earth structure collapses.
The decision to use this jutsu was made once he had sensed you sending fast chakra arrows towards the sky, knowing they'd come down and hit from above due to the angle they were sent at, the best course of decision to both avoid the newly arrows sent and the rain which was pouring down was to merge with the ground whilst remaining undetected. Moreover, due to the sudden crash of the place he had been at, you wouldn't be able to instantly realize he had hidden underground as his body might still be crushed by the rubble that was his creation.
Being able to sense your location as he moved underground, Roy decided to stop at mid range in the area directly to your right. Waiting to see what would happen, and what course of action you'd take next as you're unable to see or sense him.