WM is in back row for a reason - not to get hit as often, which means most of the team's effectiveness is coming from WM. Taking this into consideration, we can conclude that the use of her kit includes at the very least her Poison Passive, seeing as Sailor Sakura is there to touch everyone with Poison- and twice at that (thanks to Killer Bee).

Sailor Sakura is the sponge. Let her be; don't focus her. You want to focus down the initial problem here, which is the Poison. One initiated mystery focused on WM into a combo should begin to break the team down slowly. The player, seeing as they're wanting to keep the Poison passive, will change priorities into help sustaining for their WM. This makes them use their mysteries earlier than expected, changing in their style of play if they feel they are being pushed to do so.

Sailor Sakura can stall all the way to Round 10 and with double standards can be a pain, but what makes her shine the most is having access to Poison via WM.

9 Tails is here to stack ignition over x2 poison so that you can't heal as quickly. 9 Tails and Sailor Sakura are also hitting multiple units with Poison passive. Killer Bee is self-explanatory.

The ninja you have to focus down first would be WM. She's giving everyone Poison, which does indeed stack. Sealing her in any way doesn't cancel out her passive. Transferring debuffs are useless as well. You need to beat her down with raw power and combos. After she is dead, things become easier, but you are still able to be stalled out until Round 10.

After defeating WM, you want to go for Sailor Sakura next. Yes, I know earlier I said not to bother her, but you can definitely bother her now, since WM isn't there to clear any debuffs on her while she is controlled. One thing you need to keep in mind is that Sailor Sakura's Attack is constantly rising while dealing damage that ignores Defense to your entire team. This is dangerous, as it makes your back row units more susceptible to being killed off via targeting Mysteries. So yes, Sailor Sakura should be the next person you should focus, especially since you're now able to put debuffs onto her.

After you defeat Sailor Sakura, you can choose to to focus down next. It doesn't really matter. I've fought against this team in pvp and it is very dangerous and scary, but without WM and Sailor Sakura, you don't have much to fear any longer.

As for suggested teams, things become slightly easier if you are indeed MB because of Root passive, but that still won't protect you from Sailor Sakura. Have your MB use the 3rd standard so that he'll attack WM more often, since you'll be focusing her down first. Grab 2 other Sword type ninja with you who c*e mysteries in Rounds 1 and 2 (Round 1 is very crucial and you need a Round 1 combo to follow up into Round 2 so that WM is dead by the end of Round 2) and a Cleanser (ninja who can cure debuffs). I suggest either Gakido Pain or Jigokudo Pain, but if you have neither of those, then your best bet is to bring a CC ninja who can consistently control Sailor Sakura after WM is down.

If you're using other mains, follow the same formula of getting a Round 1 mystery into a combo and finishing WM off in Round 2 with another mystery into a combo. Things become easier this way for any main being used.