Lobstermen aren't that hard. Any melee weapon (stun rod or drills) owns them, and they're fairly weak to the stunning bomb. They're inherently resistant to sonic, but massed cannon fire can and will take them down fairly easily. They're dangerous, but no worse than Reapers or Snakemen from the first game unless you fall really behind on your research. Tasoths (effectively a mutons with a limited psi attack on their higher ranking troops) and (when underwater) Tentaculats (flying chrissalids!) are far more dangerous in my experience.