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

Question

OK, so I screwed up my X-Com game. Playing Classic mode. A UFO landed in the woods and I kneejerk sent 4 ill-equipped team members out to take it down, then saved over my prior save.

I've tried to take down this UFO board multiple times, but to no avail and could really use some strategic hints.

I sent 4 troopers:

1. my best Support guy, armed with a Medikit with extra uses and running; I'd love to keep him. He also has the "revive from death" ability.
2. a Sniper who I just acquired from another mission, who can snipe after moving, but is a terrible shot
3. a Heavy who is also a lousy shot and has minimal bonuses; one rocket.
4. a Rookie.

All have grenades except the Support. We tried flanking the UFO, but every time we encounter one trio of Thin Men, 2 more trios get activated within one turn. Cover is sparse; we get the occasional tree. I'd love to use someone to lead these guys back to me to take out one-by-one, but it's nearly impossible without getting wiped out, and I don't seem to be able to extract them, as there's no extraction zone, so I don't want to lose my well-traveled Support guy.

Suggestions? Start over?

Also because of my knee-jerkiness, I left the Arc thrower at home with my wounded Assault trooper. :-(

CONCLUSION: Of note, I've re-attempted using the advice below, and used the support member to draw them out. After 2 attempts, I'm keeping the successful one where only my rookie got killed; my heavy tends to panic, so he's going in the drawer.

Answer 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.

Answer 2

When dealing with overwhelming numbers your best course of action is to retreat, get high cover, go into overwatch, pick off any aliens that chase after you and retreat again if you get overwhelmed.

This may not work if you encounter several groups all at once at the very beginning of a mission and thin men are a pain because of their poison but really this is all you can do.