I have since then found out that plasma sniper rifle is the most reliable way of one-shotting mutons (far away). The downside is that it takes so big a chunk of TUs to fire (you can essentially never fire two shots), so if you're looking for total DPS, turbolaser sniper rifle provides the biggest punch. And I agree, gauss sniper rifles are also surprisingly good overall at the point you've obtained lots of power suits (with the same caveat as plasma sniper rifles: inefficient TU usage).
Also, to avoid most problems caused by mutons on open areas, I strongly suggest skipping turn 1 completely (except possibly throwing smoke) and deploying out only on the next turn. Like most aliens, mutons have aggression 2 so once they get the chance, they waste a lot of their TUs running around. So it makes sense skip the first turn crossfire with full TUs.
Now while I'm at, here's a huge tip (that I discovered myself only in this campaign) for those interested: Thanatonautian Manus is an extremely useful mid-game PSI weapon to take down armored enemies, especially heavy MIBs (always seems to one-shot them), but it also works on many others. With 90-100 PSK it is very accurate and makes tremendous amounts of damage with 0.1 armor effectiveness (with the only limitation being range 20). Felt almost cheesy using it.