300 Impulsedrive-kills can be very anoying, because the WS does about 20% of Drakesbane DMG, but it worth to do this. This is by far not the best Polearm for DRG but its easy to get and the +10% WS DMG is awesome.
Crawlers Nest seems to be a good place to do this trial.
I also did the ws damage path. I'd say Attohwa Chasm is the best place. Flys and Scorpions are as weak as EP can get and don't have any nasty status ailments like crawlers do. Just don't pull Sargas by accident.
Easy to solo your Impulse Drive kills on Vermin (or duo, since WS kills count for everyone in party) on Marids in Wajoam, since Impulse Drive will one-shot the Chigoes they pop every time, and it counts for the trial.