Countess Flank

Barbarian King

Barbarian King / Skeleton King

Since Barbarian King strategies heavily rely on the Barbarian King tanking for the rest, Pekka and Mini Pekka will take it down, whilst Countess and Archer take care of the backline. Wiz isn't needed and Healing Ranger is a maybe just in case of stun.
Against Skeleton King, you'll be expecting a fisherman who will only be able to pull in the mini Pekka. If they waste fisherman, then try to avoid the hook, but if they take the mini pekka, then leave it be. A prince will also be a likely mini for them to use, so Countess and Mini Pekka will hide behind Pekka to tank the prince blow. Archer and Wizard won't be very big with this strategy, since countess won't be targeted by the Skeleton King, and neither will the healing ranger. Pekka and Mini Pekka will occupy Skeleton King as a tag team, and countess will run around their backline whilst Skeleton King and other first row minis won't be able to do anything about it.

Archer Queen

Archer Queen / Royal Champion

As Archer Queen strategies generally involve a bunch of ranged things hiding behind a single tank, you're fine to only use Pekka, as countess and archer will pick off the enemies. Healing Ranger is recommended for the unstoppable boost, making the countess immune to any stun that the opposing Archer Queen's minis throw at it whilst it decimates everything. Wizard might be useful against a Golden Giant, but that's about it.
When it comes to Royal Champion, Mini Pekka will be a more suitable tank than Pekka, so you don't need to use Pekka at all. Archer will pick off the Royal Champion on its own, so as long as Archer survives, you can win. Royal Champion relies on the shield throws, so healing ranger will be huge here, and wizard's antiheal will be annoying for them to face as well. Countess can finish off the Royal Champion if Archer is too slow to do it, and from there their strategy falls apart.

Monk

Monk / Pink Fury

Don't play anything other than Countess for round 1. If they're playing a Monk, then have a Mini Pekka and a Pekka sitting second row in the 2 columns next to the Monk's silence. That'll make short work of the Monk, and save your Wizard or Archer until later on. If they play heals, then counter it with Wizard. If they don't play healing, then just play Archer to assist Countess. Treat Pink Fury the exact same as Monk.

Shield Maiden

Shield Maiden / Grand Warden / Natureborn

Against Shield Maiden, your most valuable mini is Healing Ranger, because it will balance out the damage you take from Shield Maiden's deflect. Because of the deflect, you'll want to avoid using Pekka and just stick to Mini Pekka. Wizard and Archer will share value here because Wizard's anti-heal will come in very useful, and at the same time, you can use its third upgrade (extra range) to your advantage. Archer is also a requirement because it needs to make the opponent's ranged minis weaker than the Shield Maiden to avoid Countess locking onto the Shield Maiden.
If you're ever up against Grand Warden, then watch their other minis. For squishies, you don't need both tanks. Mini Pekka would be better if you want to dish out AOE with the Wizard, and Wizard will actually be good for something other than just its anti-heal against this one. Against a tanky Grand Warden deck, use Pekka but not Mini Pekka, since the dissipate won't be as useful, and pure damage will be more valuable.
Even though Natureborn doesn't seem that good, it's important to stay on your toes. Wizard will need to be maxed against Natureborn, and you'll also want Pekka to counter theirs, since you'll generally see Pekka in a Natureborn deck. Healing Ranger will also be important, just in case they switch to Natureborn stun. If they play forward with Natureborn, then just use Pekka, Mini Pekka, Wizard. If they try for stun, Healing Ranger, Archer, Wizard, Pekka.

Wave Master

Wavemaster / Countess

Wavemaster Royal Ghost seems to be the only strat that any Wavemaster player is using at the moment, which is why it's important to keep everything close together to avoid isolation damage. Mini Pekka and Pekka will be spread apart so that they can tank for the others whilst at the same time threatening an early and easy kill on the Royal Ghost and Wavemaster, whilst Healing Ranger has an easy job in keeping everything from being stunned / slowed by Wavemaster.
Since Countess will target either Wizard or Archer first, it's a better idea to keep Countess from going behind all of the minis, so from here Countess can only teleport to a space that both Pekka and Mini Pekka will be able to destroy it. This means that you'll want to use Pekka and Mini Pekka, and avoid using anything else until it's necessary, or if you really need to render one of their minis useless. Healing Ranger if they stun, Wizard if they heal, Archer if they do neither.