I don’t carry pistols on missions anymore, instead I carry basic ares assault rifles with no extra ammo just to deal with mindfrages and worms. With pistol you can’t kill mindfrager in one turn and there if risk of return fire. Worms are hard to hit with pistol. Sometimes I have to fire pistol 3-4 times to hit it and with assault rifle it is usually instant kill with only 2 AP.
For the sirens you just have to disable its head and next turn it will lose control. For that you need only two sniper shots. I found that here on forum.
Paralyzed siren still keeps mind control though.