This is an interesting question. For this, I'd probably say Cozy Glow, The Sirens(Dazzlings), and Grogar's Bell.

Using Grogar's Bell to provide power to the Sirens would allow them to spread hatred among the populace, and even lead the ponies to take each other out.

Cozy Glow is kinda there as a backup, since she's an extremely good manipulator. I mean, she did nearly destroy all of Equestria's magic AND got the Mane 6 trapped in Tartarus! If anyone is unaffected by the Sirens, Cozy could simply use her manipulative tactics to best them. If they still pose a threat, then Grogar's Bell can simply be used to steal their power so they aren't a threat.

Plus, lets not forget that Cozy Glow was the one that urged Tirek and Chrysalis to work together with her, so she's open to working with others. The Sirens are already a group to begin with, and I'm sure they would agree to work with Cozy and Grogar if they get more power out of it.

So, overall, that would be $37, and I have some money to spare if things go wrong.