In multi-player, it seems that a lot of people have decided the Kunoichi is their best bet. Not a lot of highly promoted Samurai or Ronin out there, but there are LOTS of high-powered Kunoichi.
And for a while there, I was getting killed as well trying to challenge these people. The AI's routine is to run in quick, hit you with a poison range attack. Then throw in a lethargy which is as incredibly over-powered as the rest of the Kunoichi. That makes my samurai immobile, losing health, and completely unable to either chase down the Kunoichi or disengage. Even with revives, it was doing no good since the Kunoichi could recover so quickly.
Tried Ronin - even worse. And Kunoichi against Kunoichi is pointless since they've outspent me by at least a thousand GLU credits.
But there's a solution....
Heroic Mount + Reinforcement + Lethargy
The key to killing any hero is that you must reduce them to near-zero health, and then get that 1 extra kill-shot. For an enemy Kunoichi, it's nearly impossible to get the kill-shot.
Enter the combo. First, you need to have at least a medium-level Samurai. You won't be able to challenge the major Multi-player enemies without it, sorry to say. But ~300 GLU credits will let you kill enemies who must have used thousands. You need :
1) at least level 5 sword
2) level 10 farmer. max-cash assassin is good too (not necessary to GLU upgrade). Horse Archer.
5) Heroic Mount (try for at least level 4)
As the wave starts, summon the horse ASAP. You'll be riding out to see whether the Kunoichi is being covered by a shield samurai. Retreat quickly - you're not ready to fight her. With the horse, you'll live to run.
Once your lethargy and reinforcement are ready, it's time to ride.
As you get up close to the Kunoichi (the AI will welcome it), take one swing, cast lethargy, and then cast reinforcement. If all goes well, you have the Kunoichi at partial health with the first swing, slowed down so she can't run, and your Reinforcement summons a bunch of farmers/assassins right into the Kunoichi's face in a single wave. By now you should be able to take a 2nd swing and hope that's enough to charge the Legendary Strike. Cast the LS as soon as you can - no matter which option it is.
The goal is to create a storm of lethal events that the enemy Kunoichi can't get out of quickly enough to avoid that killing blow.
Now, this doesn't always work. Sometimes you just get very unlucky with the reinforcement and summon the wrong allies. So if you don't one-shot kill the Kunoichi - time to RUN (as you use sushi) and summon farmers to cover your back. Then wait and try again when Lethargy/Reinforcement is charged.
My Samurai scored at 380 in power has killed Kunoichi at power ranking 2000+ using this strategy....