Wanted to comment on this guide after literally just running through it the last three days. First big thanks to the OP for pulling together the original guide. The setup for each attempt is extremely on point and helpful to know going into it. I guess along the way there were some changes made from the Homecoming devs that fully allows the one-mission arcs to count for Flashback now, because @BrandX's missions all worked for me. Here's the ones I ran for each level range (from the Hero side):
Level 1-14: (Hero): "Rescue the Mystic From the Circle of Thorns" 1 mission...lead out
Level 15-19: (Hero): "Vahzilok Wasting Disease" 1 mission...click 3 glowies.
Level 20-24: (Hero): "I Lost My Daddy" 5 missions...Escort, Talk, Defeat 10 Lost, Defeat Muxley, Escort
Level 25-29: (Hero): A Mad Man's Council. 7 missions: 5 glowies; defeat 10 in specific zone; defeat 3; glowie; defeat 15; ambush, defeat boss mob; talk to contact.
Level 30-34: (Hero): "Defend the War Walls" 1 mission...rescue 8, defend 3 machines, kill boss
Level 35-39: (Hero): "They Call Me Doctor" 1 mission...find 2 keys, find Doctor
Level 40-49: (Hero): "The Clockwork King's Psychic Signal" 1 mission...Defeat Clockwork King
Level 50-50: (Hero): "Aura of Power" 5 missions...Talk, Defeat 15 Freaks, 15 Rikti, 15 DE (all found in Crey's), Talk
Many many thanks to everyone here for this! I remember on Live that these Flashback badges were such a slog, but this info made it such a breeze. Thank you!