It sort of depends really. I found I could generate enough guard most of the time with Fade Touched Obsidian giving me 3 guard on hit (from armor), then just Charging Bull whenever it was off cooldown. I was running Reaver, so I usually tried to shoot for around 50% health though.
In any case if you are going DPS, I would go Unbowed after Bull and grab constitution and guard upgrades from the Vanguard tree. You can potentially also steal Bear Mauls the Wolves, Turn the Bolt and Turn the Blade from Sword and Shield, but you will have to sink extra points in abilities you won't use to unlock them. Not guard exactly, but helps with damage reduction. I personally don't like generating extra threat on my 2H, at least early, which is why I avoided War Cry and Challenge (actually I took them and tried it, but didn't like it and respec'd). The upgrade to Block and Slash generates guard, but it isn't the greatest ability for that. It is mainly decent for damage mitigation if you can get the timing down, and some bonus damage early.
If you are going tank you will have better luck with pure guard generation since you can take Champion and get Walking Fortress / Siege Breaker which will generate guard, and also To The Death / En Garde which is good for larger targets.