You could've just ended your turn, Antonidas would cost 5, the girls would cost 0 and the new girl 1. Next turn, go 1) Antonidas and the two 0 mana girls for 5 mana. 2) Play Echo for 0 mana. (Cuz girls) 3) Play the girls you get from echo for 4 mana. 4) Spam infinite fireballs.
Why didn't he just keep 4 apprentices in hand, so he could Antonidus (4), 4 Sorcerer's Apprentices (0) for a total of 4 mana, and then fireball infinitely instead of almost not having enough mana.