After the patch i did both Banished Cell 1 & 2. It seems that Rilis is confined to the small stage where he is located at the beginning of the fight, and won't come off.
Banished Cell 1, the whole group was forced to dance with him in that small stage, dodging flames, and his oneshot kill, and those feasts spawns and insta heal him.
Banished Cell 2, Rilis has teleportation skill so after a while he is off stage. However, the feasts and Daedroth adds wont come off the stage, therefore pose little threat to us. We face rolled that dungeon.