I guess defense-ignoring attacks are overrated, or Flandre is way overpowered, since I've breezed my way through to F13 without having to change my initial party composition (
).
Hardest boss before F6 Tenshi was Mokou, since Flandre's nukes were rather ineffective against her (still the biggest damage I could do, go figure). Against F6 Tenshi I buffed Flande as much as I could (and Hard Mode let me) and grinded to lvl 30. Flandre ended up hitting for 30k+ with Laevetain.

F9 Tenshi pretty much got stuck in her %-based actions and died very fast, and F12 Tenshi was even easier. Since I had to try the fight several times for the drops, I went and tried to give Flandre as many buffs as possible during the fight for her attacks. On my final attempt I hit Tenshi first for 35k and then for 62k with Starbow Break, giving Tenshi only 2 turns, which were used on buffing and focus. I haven't even upgraded any stats on her in the library since 6F Tenshi.

Doing this run got me thinking: is it possible to pick a party of 12 and beat each and every boss in the game without swapping anyone out at any point (BP farm excluded to unlock stuff), post game included? And obviously doing all the fights at challenge level. There are quite a few situational boss fights where a certain character should be put in (poison/shock users for example) as well as the need to have some really tanky characters (enhanced mirror), so squeezing all the needed characters to 12 slots is not that easy.