For packs of mobs its lay down a field of caltrops and then when they are bunched up spam cluster bombs/grenades. For single target its mark for death and rapidfire (bombardment). Tumble for general mobility and for getting into position to setup a good caltrops drop.
Passives are grenadier (obviously). Archery (preference to bows for aesthetics and +15% damage). And finally steady aim because after playing with it in beta, if caltrops are in your build it is very easy to stay 10 yards away so its basically another free 20% damage.