I would strongly advise you to go for a master trait as soon as possible. Most of them are really that good.
Using the staff against lost souls is kinda suboptimal, since it deals fire damage which the lost souls resist, but if it worked, who am I to argue with it? In any case, if you feel like experimenting with special levels (and you probably should, since you don't know them all yet), be sure to have a homing phase handy to make a quick escape if things go badly... not every special level can be escaped easily...
Oh, and bulk mod on a nuclear plasma rifle is a good idea, since it allows you to shoot more before waiting to recharge.