Rex feels ashamed of himself and admits that his mind is a little bit pressured by the fact that he's facing a God, not any kind of entity but an actual God. Due to that, he lessens his mind's ability to think subconsciously.
But that changed when he heard the familiar voices ringing inside his head.
Even though he's in a different realm that belongs to Kaiser solely, the emotions of the others reach him and shake the storm clouds inside his head that blocked his mind's ability to think. Now that all of that blockage disappear, he finds that his situation is not impossible to beat.
'If I want to beat myself then I only need to do things that I won't normally do'
Although the two figures are the exact copy of him in terms of power, battle instinct, and mind, they are still not living creatures. Their abilities to adapt and learn are non-existent, they are stuck in their power no matter what happens.