No. 5 - Maiden of Sugadinti
The lead-off boss of the raid is the lead-off boss of the raid for a reason, as Maiden of Sugadinti is unequivocally the easiest of the five traditional (non-wave-based) bosses for any learner to handle.
All you really need to do is avoid the blood spots when she throws them at you, and avoid the blood spawn when it moves around the arena. Other than that, be sure to kill the Nylocas that spawn throughout the fight to prevent the Maiden from healing.
I am fairly confident in saying that you should be able to advance past this boss on your first attempt if doing the entry mode raid for A Night at the Theatre, and it’s a relatively quick one as well.