Aw shit. I forgot to post. If there's still time...
1. SA - Probably the toughest out of them all. Koishi isn't very difficult, but she has a couple tougher spells compared to everyone else.
2. LoLK - Hecatia's blue nons are shits. Lunatic Impact is also bully. Otherwise pretty simple.
3. TD - Mamizou's really fun and has some nice spellcard variety and challenge the whole way through. Nothing too difficult, though.
4. EoSD - QED is the only difficult thing about this Extra, but Flandre's only this high since it's probably the most difficult spell out of all the Extras. Maybe.
5. IN - idk where to put this, so maybe here? Mokou's not very dangerous outside of her final non, but her fight bores the fuck out of me.
6. PCB - Luckily Yukari and Ran are similar in difficulty. Good fights.
7. DDC - Even without MariB's bomb, Raiko's pretty simple.
8. MoF - Suwako's only threatening attack is Suwa War, but she's otherwise tame compared to the others.
9. UFO - I'd rate this a higher if I was judging based on scoring since the stage portion and Kogasa are bully, but since this is survival, Nue's the easiest Extra boss in the series. She has nothing particularly dangerous, and SanaeB/ReimuA pretty much decimate her.