Note that this achievement specifies Archangel
Flonne. Once you reach Post Game, there is a bill to recruit Fallen Angel Flonne. That is separate from this achievement. This is available after getting Axel.
After this achievement, there will be a new bill in the Senate, The Love Evangelist Appears!
. Pass that bill and the post-game chapter will open.
The stage itself has two geo panels, a red No Entry and a blue Enemy Turbo x3. The strategy is, on turn 1, get a long range character to destroy the No Entry panel, then throw more characters in. You want to lift Flonne off the geo panel with one character, then either destroy or throw the other geo panel out of the area, then cancel the Flonne lift to put her back down and conserve your action. From here, it's largely a normal stage. Enemies are level 150, with Flonne being 200.
If you're still struggling, you need to power level some more. At this stage, your best options are 5-2 or 9-4, as you can easily abuse the level 99 glitch (a level 99 enemy gives XP and mana of a level 323 enemy) to power level.
5-2 is for units that can hit at least a 2x2 ranged area. Set the enemy level in the cheat shop to 8 to make them all level 99. First, go to the bottom right of the screen and move the yellow geo block onto the rest and destroy it to make the entire area EXP +150%. This is an excellent area to level Valvatorez with Tyrant Flughude, but the cockatrices have a silence aura, so you'll need someone ranged there. If you magichange a monster into a weapon, the EXP is copied, not split, to the monster as well. Stats don't matter, so a level 1 monster will work fine.
9-4 is strictly for monsters and Desco. Set the enemy level in the cheat shop to 1 to make them all level 99. Use Valvatorez and Fenrich to throw the geo panels to give EXP and Mana +50%. Move Desco up and any monster at all right behind her. Use that monster to fuse and make Giant Desco. The main advantage is that Yog Sothoth now hits a 5x3 area, meaning Desco can one-shot the entire group. If she's struggling with power, her passive boosts her ATK with adjacent allies, and Male Fighters have the ability Fight Shot to double ATK for 3 turns.