The build Lolwut posted at the top is fine pre-rebirth. Burning Land is one of my favorite moves since it has low cooldown, great damage, and "resets" by being able to hit downed targets. Only downside is you can be in trouble if you use it at the wrong time.
Amplified Railgun is good enough at 1, so yeah. Concentrated Shot is fine pre-rebirth, since all you are doing is regular stages where it is actually effective. Elsewhere, it isn't so spectacular.
Rapid Shot doesn't pierce, but the rebirth version does, so just wait until that. Diversion Shot, I thought about messing with since the bullets do pierce, but I'd rather use it as utility. Physical Damage passive and Merciless Shot are not worth pumping.