Some Colossi, namely Molten Fortress, Gluttony, and Frost Walker all have complex mechanics, requiring high levels of teamplay to beat. Usually, if even one squad member does not know what theyre doing, it leads to a total party wipe. Players complain that theyve tried dozens of times to beat a boss, but always end up losing, due to other players.
Source: N4G PC The First Descendant Devs Tease Some More Incoming Colossi Nerfs and Changes