As the shield vanishes, Strange gestures with his hand and the spike of earth retreats back to the ground as well. If Reira says she's done, then they're done. And he'll offer an honest assessment.

"To be frank, it is a good shield. But none of the magic I cast was anything close to full strength." There's an unspoken 'and so, it'll be useless in an actual fight' but Strange refrains from saying so. "More practice with the shield should increase your stamina and make you more used to the magic. And when you practice, the practice should be closer to what we did instead of simply casting the shield."

If Reira wanted to defend herself against things, then she had to practice...well, defending herself against things.