Cellbind
Skeletron
@Sigma90
You can in fact challenge Tapu Koko again after that initial scene, without even fighting the elite 4 again the first time. I skipped the fight during my second play through, and was able to return just fine.
If you flee or defeat him a second time I think you still have to proceed through the elite 4 again, but I'm pretty sure you still have as many chances as you need to catch the legendaries.
If you flee or defeat him a second time I think you still have to proceed through the elite 4 again, but I'm pretty sure you still have as many chances as you need to catch the legendaries.