This is how I play the Katari. I hope this helps at least one person. You do not have to play him this way. If you do something else, that is ok.
Active Skills
Charging Bull
Mighty Blow
Earthshaking Strike
Pommel Strike
My unlock order:
2 - Clear a Path - Str+3, hitting multiple targets = 10% stamina per hit
3 - Mighty Blow - Active Skill
4 - Pommel Strike - Active Skill
5 - Gore and Trample - Charging Bull upgrade
6 - Warrior's Resolve - Const+3, 10% stamina restored for every 10% health lost
7 - Flow of Battle - Str+3, criticals reduce cooldowns by 1 sec
8 - Opportunist - Cunning+3, 10% critical chance when attacking from behind
9 - Earthshaking Strike - Active Skill
10 - Easy Target - Mighty Blow upgrade
11 - Shattered Ground - Earthshaking Strike upgrade
12 - Fervor - Cunning+3, 30% damage bonus when enemy dies within 10m
13 - Adamant - Const+3, Armor bonus +20%
14 - It'll Cost You - Str+3, 15% damage returned to melee attackers
15 - Bear Mauls the Wolves - Const+3, immune to flanking (mandatory)
16 - Lighting Jab - Pommel Strike upgrade
17 - Coup de Grace - Str+3, 30% damage bonus to knocked down/stunned enemies
18 - Relentless - Cunning +4, Str+3, Const+2
19 - Scenting Blood - Const+3, 50% movement speed/10% crit chance >enemies 35% health
20 - Blood Frenzy - Str+3, 5% damage bonus for every 10% health missing
Playstyle
First understand what you are. I am an offensive weapon with a practically unlimited stamina pool that I am going to use to charge in and spam 1-3 offensive action skill strikes. I do not have a shield and do not ever intend to let any enemy hit me. Any enemy within melee range should be knocked down, stunned or dead. All of my skills have low cool downs. Successfully charging gives me a 6 second window where I can use an offensive attack without costing myself stamina. This allows me to ignore stamina and spam my attacks as fast as possible.
Every attack begins:
Charge > Mighty Blow > Earthshaking Strike
which I then have to decide whether I want to Pommel Strike > Charge or Charge > Pommel Strike. This all happens in the first 4 seconds of each encounter. After the initial attack, I then continue to charge and rotate my offensive attacks depending on which one is on cool down.