Keep in mind that to effect a drone with powerbolt, you need to beat its object resistance which is at base 6, but can go higher. To average 7 hits, you need a spellcasting pool of 21 dice, which can be tough (impossible?) to get.
That's why I'd advise taking a non-direct spell to combat drones. You don't need to combat their OR that way. I also suggest taking an elemental type as those get to ignore half of the target's armor and have nifty side-effects. Electricity won't actually damage the drone unfortunately, but it can knock it out. Fire can be nice, as could something like sand, which would have a chance to gum up the drone's works.