[Auxilism]

Yes, Nanoha was trying to improve Teana's shooting but she might not have expected her to go to such extremes. Teana could ignore all safety precautions in order to maximise the damage her shots would do even if they could kill. Or she could take the longer route to strengthen her magic slowly so that it would be safer for her yet inflict the same amount of harm without killing. Nanoha also said to her, "shooting magic can hurt" or something like that, inferring that it does not have to penertate the skin to do its job.

She also admits that Teana was sick of seeing little or no results from the training, which would have caused her to take 'shortcuts' since she wanted to prove herself. We already saw that Teana could not control multiple shots well if she overloaded them when one of them almost hit Subaru. Maybe she learnt from that and chose to concentrate all her magic into a single shot (or blade) so that she would have more damage and control. Of course, this increased the danger factor and Nanoha decided to step in.

Her 'most attractive trait' may be her multiple target shots, rapid fire or her charged shot with one gun. Nanoha only blasted her when she started to use Phantom Bullet, which had both Cross Mirages charging a single shot. It might have been on the scale of a Divine Buster, a heavy bombardment type attack. Since Teana has never used bombardment spells before, Nanoha would have thought it would be too dangerous for her to start with such a strong one.

Then Nanoha showed her Cross Mirage's mode two, Dagger mode. She explains that she will be taught the other styles of fighting but in good time. She only showed the melee form because she did not want Teana to try the bombardment form yet, Blazer mode. Bombardment spells would still be too powerful for her since she admits that she would need more practice with it after RF6 was attacked.