I think it's less of Rairen getting stronger, and more of just none of Kyouka and Yuuki's team not being physically strong enough to harm Rairen. Even when Rairen first showed up, most attacks barely had any noticable effect. It wasn't until Tenka joined the battle and used her spatial attacks (attacks that ignore most, if not all, external defenses) that Rairen began to take immediate damage.
So, long story short, Kyouka and Yuuki's team are going to have to do more than simply punch, kick, shoot, and/or throw rocks at Rairen before they start dealing any noticeable damage.