Yay, just played my first multiplayer game and won.
We played on the self-explanatory level "Bar". I went with a nice varied alien squad: a sectoid, two thin men, a heavy floater, and a muton elite. My opponent went with two extremely highly upgraded xcom soldiers (their armor was immune to my poison spit, and they had plasma rifles) and a support force of three sectoids and a floater.
The first casualty of the round was my thin man. He met a floater on the roof, causing my opponent to rush an elite soldier onto the roof and blow him away. My heavy floater missed what should have been a fairly good reaction shot too. Anyway, my heavy floater ignored the roof and lobbed an alien grenade at the corner of a bar, wounding one heavy soldier and killing a sectoid. A sectoid tried to suppress my muton elite; except the car he was using as cover had been clipped by the grenade throw and exploded. I gunned down the xcom soldier on the roof with a (very likely) crit from my heavy floater. Next turn, they brought all their forces onto the roof and shot down my heavy floater.
This left me with a sectoid, a thin man, and a muton elite to their sectoid, floater, and xcom elite. There was only one ladder and I was pretty sure they were overwatching it with everything, so I moved everything but the sectoid to the side of the building. That poor runt got sent up the ladder, and took three plasma reaction shots at close range. But I knew where they were now, so I had the muton toss his grenade into their cover, killing a sectoid and reinforcing the xcom soldier's newfound fear of alien grenades. Then the thin man leapt onto the roof and fried the human scum at close range. The floater, sensing defeat, didn't even both moving closer for its shot, just dejectedly missed the thin man from where it was and then got gunned down.
Its really fun. Oh, and don't be afraid of overwatch; free aimed explosives, lightning reflexes, ghost armor, squad sight snipers, evasive units, expendable units, suppression, luck, sticking to cover, or just not moving into enemy sights all counter it.