For dungeons, I run Rifle + Axe/Warhorn, with a primarily glass cannon build. There's definitely a better offhand for that, but warhorn's what I have and I keep it around for the swiftness buff while leveling. I bring the heal skill that removes conditions (lifesaver that one when mobs stack conditions), and for utilities I bring For Great Justice, On My Mark, and the Signet that grants Precision/fills adrenaline bar. My Elite is the banner, I bring it mainly for the instant rally effect. My spec is primarily into Arms for the extra precision; I have a decent mix of Power, Precision, and Crit damage on my gear. In arms I grab Crack Shot and the vulnerability on crits trait. I aim to keep vulnerability up on my main target to improve my group's damage, and I stack as much vulnerability as possible to improve my Killshot and Volley damage.
For sPvP, I use a different build. I use the same weapons (Rifle + axe/warhorn) but I build much more balanced. 25 points into Arms, 30 into Defense, and 15 into another tree which I can't recall atm. Pick up the same Arms traits as above. In defense I grab the traits to activate Endure Pain when I hit 25% health, activate the Stability stance when hit by hard cc, and gain adrenaline on being hit. I take Runes of the Soldier to gain the remove a condition on Shouts passive, to ensure I have some condition defense. I bring the heal which increases with adrenaline level, and for utilities I bring For Great Justice, Fear Me, and Shake it Off. Signet of Rage for the elite. Basic strategy is to apply vulnerability to targets to improve everyone's damage on the target, and use my cc to control their movement. Rifle 1 > Rifle 5 > Rifle 2 > Rifle 4 > Rifle 3 > Fear Me > Weapon Swap > Axe 3 gives me enough time to flip some points from full capped enemy to full capped me, while dealing significant damage to the defender. Will get me at least a neutralize against almost any defender (and, indeed, can often get me a neutralize against multiple defenders). Depending on how long it takes them to get back to me before I have to pop Fear Me I sometimes don't even have to weapon swap to get a cripple. I use Killshot whenever I feel like my opponent is unlikely or unable to counter it, such as right after they dodge roll multiple times or when they don't seem to be watching me during a multiple person fight.