All we actually have to do is hire one international assassin! (International= professional+far away= once they leave, unlikely to get caught) And then, all we have to do is hire some thugs (anonymously) to destroy and disperse her remains to make it slightly harder to resurect her.
All we have to do is take advantage of the lack of common sense in this type of setting (i.e. Lack of snipers hired) to pull off an "unorthodox" attack that should turn out relatively untraceable.
Especially if we hire the sniper anonymously.
And we have to do it as soon as possible. The longer we wait, the more power the MG's have a chance to accumulate. Good guys or not, they really are like an infestation. And you don't deal with termites by letting them reach the numbers needed to eat the house before you call an exterminator because "something else might eat them and save me the money."
Also, one of the most dangerous groups is the Choir. They are relatively weak, but highly experienced. And while the Princesses are likely to have a while before gaining attacks that home in on people and identify them, they are fighting more people every day, encountering new things such as a Youma who possesses people. They are growing no matter what. We need to limit that. Killing Heart means they have no way of stopping possession, removes the member who really holds their team together, and is killing a member who likely never did much in fights anyway, so they won't learn many new tricks to make up for her absence.
Destabilizing the team at this stage in their growth allows a strike at the beginning; namely, before people learn to trust them, they are framed as the villains. Whoever is summoning actual Youma (noted to be an extremely powerful summoner to summon their Physical Forms, instead of possessing spirits) will likely play along, summoning tons of possessed with specific instructions.
While some more genre-saavy people will figure it out, the Princesses are on bad terms with the authorities because they openly state an intention to conquer the world. They are tolerated, Word of Questmaster, because they keep the Youma from doing whatever they want. If the aforementioned Youma seem to be their fault? The authorities will want to imprison or kill them. After all, a frame-up the masses believe is cliche, and life isn't a story, so surely this must be real!
Later, have tons of possessed gather in a square and beg for their masters' (mistresses?) help. Have them recieve a "sign" based on the 5 Elements left, and take this as a signal to start destroying the city. When the Princesses give in to their MG instincts and show up, have the Youma obey them, except when commanded to tell them who they work for. ("We serve you loyally, Princess Earth! We swear we have betrayed you to no master! Look! We will gladly destroy this city we once protected, as you have ordered, to show our devotion!")
Have the Youma seem like stupid minions who have been brain damaged or empty charmed- some reason for them to forget this is a secret. Maybe have one Master Youma arrive, apologize to the Princesses, and explain, since their cover is blown anyway, that they were in an (experiment/attack/write-in) that caused this, and that his associates are not trying to ruin the Princesses' plans. Then he begs them not to kill them.
Eventually, a Princess is going to command a Youma to leave the body it is in. The Youma will look thankful, and say, "Yes, Princess. Thank you Princess."
After that, arrange a car accident or something "accidental" to remove the Princesses when not powered up. All at once, in a believable fashion.
Then spread rumors (especially on the internet. Anonymous, and can spread info fast. Fantastic for control of the Rumor Mill). The Princesses have vanished to perpetuate a new plot. Those people died, and it is sad. Also get a second email so that you can set up a different set of anyonymous accounts on forums and the like. Start rumors about how those people were really the Princesses, and how they were framed. Make it Black-and-White, so people choose one or the other. Than use poor logic and an abundance of evidence from stories, used poorly. Use Victor's social skills to play people's need to believe that life isn't a story, and equate the argument that the Princesses were those people who died with life being a story.
Than create a third email (computer, Gmail, Yahoo, Bing, Personal) and set of accounts and argue that the Princesses are really alive, and those poor girls, and that they faked their deaths. Split it between "they were evil" and "they were good," note how story MGs would never let their parents think they were dead if they were good, etc. That should split up the crowd, and the accusations against people's now dead friends should allow some interesting social opportunities on Victor's end at school.
There are so many opportunities created by the death of Heart and removal of the most powerful, likely most naive MG group- the new one with the most potential. Not to mention that at the beginning of the thread, basically everyone's plan was "assassinate the Heart." Now she's named, and there's a number of benefits of doing so, people aren't aren't focused on doing so?
Aaaaargh!
Also, apparently killing Heart and dispersing her remains to prevent "miraculous survival" is seen as excessive?
I still have an issue with that one.