Sneak attack, of course, is one of the best around. Find a way to always target flat-footed AC (greater invisibility is good), and you'll do loads of extra damage.
Seance Boon gives a day-long buff to non-spell or spell damage, depending on which spirit you or your teammate channels. This can be scaled up by various FCB options like halfling's or shabti's.
This may be cheesy and require some GM approval but you could use the cantrip to throw an alchemical splash weapon like alchemists fire. I know the spell states that the type of object thrown doesn’t change the type or properties of the attack so it will still do 1d6 bludgeoning damage but it also states that the object thrown also takes this damage which will surely break the flask and douse the target with its contents as with a direct hit from a splash weapon would. Should you miss though, the target won’t take splash damage since you aren’t lobbing the flask but flinging it hard enough to do physical damage and would likely fly on past the target up to the range of the spell or until it hit a solid object like a wall. IF your GM allows this you could take a level of alchemist to add INT to damage and/or you could take the Concentrated Splash feat to increase its damage by 50 percent. On a side note you could take VMC wizard with the Evocation school to add 1/2 level to damage of the spell.
Composite Bows add your Str to the damage. 3 levels of Trench Fighter lets you do Dex to Damage with firearms.