Question: What good do you think aggression is going to do?
You can't intimidate Crusty, because he's IMMORTAL. Sure, Flandre could Wipe the whole five-man party by herself... this time. But the Adventurers are like fairies -- they don't stay dead, and there are SWARMS of them. Except unlike fairies, Adventurers are strong and focused.
Eventually, there will be party after party of Adventurers gunning to "clear" the dungeon that even Berserker Crusty couldn't beat. Or Crusty will decide that this was actually a RAID dungeon, and bury the place in DDD's top players. Over, and over, and over again --- each defeat simply proving that there must be some kind of awesome reward for killing the Scarlet Devil.
Meanwhile, the Scarlet Devil Mansion can't replenish its numbers. The humans will stay dead, and the youkai resistance to permanent death (recall that they generally have to be sealed) may not work right in Seldelsha. Even if it does, youkai probably take much longer to resurrect than fairies and Adventurers.
The Gensokyo humans will also abandon the mansion to go join human settlements which AREN'T ruled by youkai or constantly under attack by "youkai exterminator fairies from Hell".
If you can't intimidate Crusty, maybe you assume by being aggressive you can shame him into peacefulness?
Shame Crusty? The Berserker who openly lusts for blood? The Glasses Villain Youkai who mercilessly teases Renessia for fun and profit? The Crusty that religiously played an MMO in a culture that looks askance at anyone with such an insular hobby? You mean to shame THAT Crusty?
Moreover, what does he have to be ashamed about? He's just raiding a Dungeon, fighting programmed monsters. He's doing the world a righteous service by invading a vampire's home, killing everything he sees, and making off with the loot.
Don't antagonize the Adventurers. It will doom the Scarlet Devil Mansion.
Flandre needs to make friends with them.