How do I defeat these 12-15 Thin Men and the commander?

Solution 1:

A few recommendations:

1. Take the high ground. Most maps have a higher area with a decent amount of cover. Every point of higher elevation means more accuracy and defense. Try to find the high ground on your map and set up your squad there.
2. Expose and retreat. use your medic to expose 1 squad by taking a half move, then retreat back out of range of fire. He should be pretty safe since he can move extra far. This draws the enemy in without (hopefully) exposing additional squads.
3. Use your rookie as bait. Position your rookie in full cover somewhere close to the enemy and then toss your smoke grenade at him. See if they are all willing to shoot him instead of going after your other units. The added benefit of this is that your rookie will now be in grenade range so he can weaken or kill a few of them.
4. Conserve your healing. Don't use it immediately to cure poison, wait until your soldiers are near death before using your med pack.
5. Use explosives, all of them. I figure, your rocket alone should take out 1 group of thin men. 2 grenades should take out another group. So that leaves only 1-2 groups left. The UFO boss is always in the same place and is easy to gank if you position your soldiers accordingly before exposing him. If you have a single grenade left, you can easily blow up his cover and take him out in the turn you expose him.
6. Cheat. If all else fails, you can 'cheat' by figuring out where the enemies are before exposing them and then rocket that area to hopefully take out multiple thin men groups. Or you can set up ahead of time and throw in 2 grenades as soon as the battle starts. This is assuming you aren't playing on iron man mode.

And always to remember to spread your squad out against thin men so they can't poison two people at once.