So, I have a SMN friend that mentioned to me that he saw somewhere that if you level over 20, you can't do the solo avatar battles and instead have to do the party bouts to unlock avatars?

Is that true? Or does Nasomi work differently? I figured it'd just sync you down to them if you were over 20, but I wanna check with the community to be sure.

99% sure you can do it. I was a 22 SMN and went ahead and did the solo battles to get the summons.

This was about 3-4 weeks ago and I have not heard any of any changes.

The mini battles work and can be accessed at any level 20+, but you don't want to do them at all unless you can't find anyone to assist you with the prime fights. If you do, you lose the ability to teleport directly to the cloister while on SMN for any future fights/convenience. It can be very handy later on at 75 if you need to do something in the more remote locations such as Fei'Yin or Den of Rancor to just hop on SMN and teleport directly there from the quest NPC.