Main point: In Perilous with Katari you are not going to build guard to take hits like you can in Threatening.
Obviously you can still build guard, it just doesn't do much.
I keep Charging Bull because it's great for mobility, the extra 25% health from full guard is better than nothing, and the knockdown is sacred. I do not use it to enter combat, I use it for knockdowns while I'm in there, usually just positioning myself to knock down 2-3 guys in a short dash.
I also keep mighty blow, I'm really comfortable with it's range and timing because I use it on my Reaver also, and knockdown is knockdown. An enemy that can't attack is the best mitigation a Katari has, the Damage is also very good against targets you already knocked down with Charging Bull, and if you stick to short runs. With practice it becomes second nature to usually charge 3 closely grouped mobs, knock them down, then turn and Mighty Blow all three.
To the death, you use it, you know what it does. I also like it.
And where I differ from a lot of Katari I see, but something that's catching on due to it's utlity for interrupting, and LOL factor for ruining the Demon Commander's AI, is Grappling Chain. It's very broken, there are a some targets it won't work on at all (wraiths), and it has about a 30% fail rate on the damage and the stun from the upgrade, yo do the animation, and it just... misses. But it's so much fun.
I honestly don't think that whirlwibd is worth it, tbh.
This.
Pommel Strike and War horn are both sensible and worthwhile alternatives to Grappling Chain, btw.