So I remember back when I played alot I found a way to solo kyuubi with Gai at lvl 8 and I would always make this my goal to do whenever I got Gai but ironically in this guide I'm holding off on Gai solo strategy until I find most reliable/optimal one. But I have 2 other heroes who can do it quite decently and being able to kill kyuubi at relative early level is pretty huge because when he dies you get 2 1/2 levels and gold from kyuubi kill +1k gold from kyuubi essence if you choose to sell it. Thus making it worth it.
Lee
What you need:
-Lvl 8, although 9 is optimal.
-Items Mana boots, yes I said mana boots I will explain later, Mask of Madness, Shin Guards any other items you after this are luxury and will just make it easier.
-Skill Build 0-4-3-1 if lvl 8. 0-4-4-1 if lvl 9
Skill/Item Reasoning
The way this strategy works is you need your drunken brawler skill maxed for the 75% evasion and the mana boots to be able to spam the drunken brawler skill throughout fight. The mask of madness is also needed for dps and speed purposes. Your ulti is kind of useless because it uses up too much mana and kills you too quickly the mask of madness uses less mana and gives you faster attack speed.
Strategy
Go to kyuubi activate drunken brawler and mask of madness. Whenever your drunken brawler tooltip dissapears you must run/kite him around till it's off cd(relative short cd of 12 when maxed) Whenever your drunken brawler is off cd you should be using it and attacking kyuubi all while activating your mask of madness when it's off cd but only if you have drunken brawler on too. Do not forget to use your mana boots when they're off cd too this is important. Keep doing this until kyuubi is dead also towards end when he in red zone you can just yolo ulti him to end fight faster.
Sidenote fight takes 1-2 mins tops and no one checks kyuubi in pubs anyways unless they have maphack and see what you're doing "then randomly happen on you with their whole team"
Also try to abuse runes, runes weren't used for sake of this guide but if you get Double Damage then EZ kyuubi essence.
I've done this 3 times with no problems he doesn't even come close to killing you, only thing you have to worry about is managing your mana and keeping eye on drunken brawler tooltip so you arent taking unnecessary damage when it's off.
Will Post Juugo/Gai Guides soon.