How effective is a BTB + Polaris combo? Should I aim for that or just stick with BTB + an elemental sword instead?
2 slot weapons suggestions
That should work. The only damage you're not covering is shadow, but with two different damage types, it should be decent.(Although, I would really try shooting for weapons slots upgrades, so you can have a more varied line up, but that's just me.)
The Polaris is a nice gun, it can push back some hoards while you weaken them from a distance. (This isn't saying the elemental swords are bad, but I feel Polaris might be better at pushing back some hoards.)
So...yeah, what you have sounds good. :)
You might want a bomb too, for crowd control. Polaris doesn't work well when surrounded, and BTB doesn't work well on constructs that tend to crowd you (gremlins usually spread out more, and arenas almost always have hordes of mechas)
I personally hate, hate people whose only viable weapon is a Polaris. It's a very situational gun, and when you have 4 in your party it can be epic, but more often than not, all it does is knock attacking mobs into me, force mobs I'm attacking away, and blind me. Sure, you can do your best to use the non-expended shots, but with the ways mobs are knocked around in parties and how slow the bullets are, you're bound to miss quite often and have the expended shot bother unsuspecting people. I used a Calibur -> Levi as one of my weapons when I was using 2 slots only. At least that way, you can be useful against all damage types and save your gun for situations where knockback is good to use (I love herding lumbers into corners with mine). If you want to stick to the BTB, get a gun that isn't so inconveniencing for others, perhaps an Alchemer. If you want explosions still, Biohazard might be interesting to use, though elemental guns are more useful most of the time.
"...all it does is knock attacking mobs into me..."
"...I used a Calibur -> Levi as one of my weapons..."
Which is just as bad as Polaris if used improperly. Worse because it pushes mobs in ALL directions instead of a straight line back.
Depends if you want to party, others can pick up the slack in Construct/Jelly strata meaning that this combo is fine. You can also turn the Polaris more party friendly by getting up close, it does less damage but since the bullets don't explode before traveling a certain distance you won't get complaints about knockback.