The way I clear this stage is relatively straight forward, but it is a really long fight. Your Malroth needs to be able to move faster than the Ethereal Serpents and sleep and envenomate all three of them on turn one, that is the only point of RNG in this fight aside from Dragurn getting paralyzed which rarely happens assuming your other units are strong enough. One other note in particular is to pay attention to killing Cosmic Chimera with poison. After you put it to sleep, you need to move Malroth down on the next turn or Kaleidocat will kill him. Then, the turn after that, Malroth needs to deep sleep Cosmic Chimera so poison kills it. If you kill Kaleidocat before Cosmic Chimera, Leery Lantern will start covering Cosmic Chimera and then you cannot do enough DPS with this method to win the fight because Cosmic Chimera will keep healing Leery Lantern.There should be a better way of doing this using Marquis De Leon once he is available, and Metal Dragon, I'll try this and upload another clear of this stage showing that method once I have the unit and level him.