Ran's spellcard experiment causes nothing but a stare; not even the recoil she saw in the vent. It does distract it long enough for Rex's punch to connect however, bits of red ichor splattering against the wall! And... that splatter starts to coalesce the moment it does, pooling at the bottom.

Meanwhile the half he's still fighting is still a bit larger than him, and throws itself at Rex to pin him to the ground.