Ah, that's the one I usually do last. The other ones, the Geth are set up in a fashion where the blinky flashlights practically beg you to set up camp on the high ground within rifle range of their base and snipe them into oblivion. Ugh, thresher maws. On my first playthrough I tried methodically killing every single one in the game (9, I believe). That campaign lasted for three kills and then after that I decided I'd just shoot up the one I had to for "UNC: Missing Marines" and leave the rest of the worms alone. Those are still the most loathed enemy units in the entire ME series as far as I'm concerned.