Don't really know, but I thought Charge - Whirlwind - Grappling hook with the kick where it hurts + one more ability yet to decide on might be fun. I had lots of fun with charging bull alone when I tried him first yesterday.
My idea is chaining Charge and Whirlwind. No idea where the grapple thing fits in but I want to kick stuff in the privies. Not sure about the war horn - it might fit the general theme of the Katari Express Freight Train, but I rather would see it activate during the charge. Maybe I'll try out a build with dodge - sometimes you just have to derail a wreck.
I'm on my third play through with the Katari.
Whirlwind is "fun" but ultimately you end up spinning around in the middle of the battlefield not being to terribly effective getting shot at from all directions. It is nice if you can combine it with a skill that groups enemies together. However, I feel like there are better offensive moves the Katari has than Whirlwind and the skill slot/skill points would be better utilized with a different skill. But by all means, use what works for you.
Mighty Blow does BONUS damage to knocked down enemies. Charging Bull knocks down enemies. Charging Bull > Mighty Blow is VERY, VERY, VERY effective. IF you have the Gore and Trample upgrade, this cast of Mighty Blow does not cost you stamina.
Earthshaking Strike is a powerful AoE attack that hits twice, once on the initial impact and then again when the fire erupts in a 12 m line which also puts a DoT on any enemy that walks though it.
You charge in with Charging Bull, knock enemies over, mighty blow them on the ground, follow up with an earthshaking strike. At this point you are probably surrounded by enemies, with only a little bit of guard from Charging Bull. You are out in the middle, exposed with Charging Bull-Mighty Blow-Earthshaking Strike on cool down. Use War Horn to make the enemies around you to panic, not attack you during the 3-4 second window in which you are exposed while you wait on the 8 second cool down of Charging Bull. As soon as it comes back, Charge into the nearest remaining enemy for a little more guard, to knock them over and finish the group.
I find this very, very fun.
I have an HD PVR. When I get my Christmas Bonus I plan on buying my gf a laptop. When this happens, I'll be able to record my gameplay. I'll make a video. Give me a few weeks.