Here are all of the bosses (Spirit Ash = Boss):

Black Knife Tiche = Romina, Saint Of The Bud
Banished Knight Oleg = Twin Moon Rellana Summon
Banished Knight Engvall = Putrescent Knight
Latenna the Albinauric = Golden Hippopotamus (Can be Buggy, make sure to summon it on a clear area as it might fall through the map)
Nomad = Messmer The Impaler
Crystalian = Midra, Lord of Frenzied Flame (1st stage only)
Ancestral Follower = Midra, Lord of Frenzied Flame (2nd stage)
Cleanrot Knight Finlay = Commander Gaius
Fire Monk = Furnace Golem
Kaiden Sellsword = Promised Consort Radahn
Blackflame Monk Amon = Meytr, Mother of Fingers (Might be a little weird with it spawn and push you back)
Kindred of Rot = Divine Beast Dancing Lion (Might be a little weird with it spawn and push you back)