what's the least painful way to get rid of german roaches?

Gel bait, not spray. Spray just scatters them into the walls and if it gets near the bait they won't touch it. Put small dots of gel in the places they hide - hinges of cupboard doors, under the sink, behind the fridge and cooker, round the dishwasher. A few sticky traps help you see where they're thickest. Then take away the competition: no crumbs, no grease on the hob, no dripping taps, so the bait is the best meal in the kitchen. Give it a couple of weeks. If numbers aren't dropping after a month, switch to a different brand of bait, they can go off one. And if you're in a flat and they're coming from next door, that's when it's worth getting pest control in, because you can only do your side.