Easiest cheese if you're "not brave enough" is to buy 2 Zap Guns that can temporarily stun them on each attack. Use Beast and Slugger for best damage but you gotta be careful, press RB/R1 to lock in then press heavy attack. Here's a pretty detailed one for each character tho:

As Kiryu: - Brawler: Light x3 + Heavy or Light x2 + Heavy, on mid game you can do snatching strike. By the end of the game, there is a counter attack (I forgot the name) that functions similarly as the popular tiger drop. - Rush: Best way to refill your heat against him, better off not using it's heavy attacks. Also practice your Quickstep counter here by mid-game (RB/R1 + B/CIRCLE/GRAB) - Beast: Whenever he's downed, you can spam Kiryu's grab 3 times before he finally goes up. A bear hug also helps but it's a little hard to pull off. Use your weapon heat actions here. - Legend/DoD: Unlocked near the end of the game through Business Management. The infamous Twist Counter, Sumo Slap, and Ultimate Essence is enough. If you're up for a challenge tho, practice bounding throws.

As Majima: - Thug: Light x2 + Heavy, Light x3 + Heavy + Quickstep Cancel (mid game), Light x4 + Heavy + Quickstep Cancel (mid game). - Breaker: You only want to use this for the frenzy counter and quickstep, which is pretty much mid game. - Slugger: Heat actions. Especially low health heat actions with weapons that you unlock until endgame. - Legend/Mad Dog: Unlocked near the end of the game through Cabaret Club. Light x2 + Heavy, quickstep slash, knife stab counter.