This must be a bug. You are actually on a platform when you respawn. Frequently Nihil will launch a shard while you are still on it. It will land on the platform & you can retrieve it from there even though you can't see the platform you are standing on. It needs to be quick though - before the platform despawns. Multiple times the shard was in the air already as I respawned and I was able to pick it up, throw it, and get off of the platform before it despawned. (The platform still changes colors before it despawns.)
Once you hit stages 2 & 3 when he starts teleporting all over the place, you just need to move in the opposite direction. If you are on a platform across the playing field when he arrives, he will almost always launch another shard at you. No sword swings to avoid at all. Though in stage 3 he will sometimes teleport multiple times without attacking between them.
Not really. When I finally completed this one, there were only 5 small platforms left. I was primarily hopping between 3 of them trying to get enough distance for him to throw shards. His last 2 bits of health took about 2 minutes because of the near-constant teleporting he was doing before throwing shards.
Now that I have the Vitrica, I will not be playing this one again. There are faster and easier ways to get rivens.