It is impossible to answer this due to the lack of infos but I have two theories:
- Enryu is no stranger to the Tower. He might have been around since the creation of the Tower. He could even have more privileges than irregulars as the messanger of God.
So he is not a newbie at all when it comes to shinsu control. We know that he is better than an Admin but he still needed a long fight to finish it off. And we could argue that Enryu didn't kill the Admin because it reincarnated itself.
- shinsu means divine water. We can think that it's a power that belongs to divine beings such as the God of the Tower or the God of the outside. (They may or may not be the same being).
Enryu, as an emmisary, could be able to tap in this source of power.
If you take the extended univers into account, Gods could be Axis and shinsu Axis's field. But when used with Axis's level of skills, shinsu can become something that looks foreign to the shinsu that inferior beings know of. (Cf Phantaminum)