The skills bar disappears but that does not mean that you cannot complete the episode since all of your skills still work with the standard keybinds. Also, Peitha will revive you if you are ever downed.
When Cerus gets to 75% and gains the Impenetrable Defence buff, you just need to press your Special Action Key to remove that buff and spawn a pair of demons. Defeat one of them and then continue to attack Cerus until 50% when you just repeat the same SAK, kill demon, kill Cerus steps again (and again at 25%) and then use SAK at 10% and at 5% to defeat Cerus and move to the next step.
You can do the whole thing using just auto-attack if you want.