All human XCOM teams are unbeatable in MP. Even just one max level sniper with good gear can massacre entire teams of aliens on his own.
The rare times I go MP, we always agree no levelled humans.
One Ethereal, with as many rookies or sectoids as the remaining points can get. Hide the Ethereal. Use the cannon fodder to find him, then RIFT his punk ass. I did this and won against an all-human, high-leveled team.
Ethereals cost 10500, too much for standard multiplayer matches. With good reason; any MP match that included them would become entirely about them. Not really fun.
Really there are two (well, 3 now) seriously OP things in Xcom multiplayer.
1. Xcom troops loaded up with a number of small upgrades.
2. Squadsight snipers, just like single player. But especially because the standard sniper loadout includes opportunist AND damn good ground, for some unfathomable reason.
3. Maybe MECs. They seem heavily underpriced for all the utility they bring to a team.
If you're wondering what I mean by #1, look at it like this. You have a rookie, that's 800. Now make him a smokejumper support, adding 2 health, minor stat improvements, and a smoke grenade for 100. Now give him a light plasma rifle, that's +10 aim and crit, and +1 damage, for 250. Now add a flashbang, letting him cripple a more powerful opponent for 100. Then add the watcher genemod, adding the ability to leap tall buildings in a single bound, and +5 aim/crit for doing so.
Grand total, 1350 points. 7 health, 5 damage, great accuracy and good crit chance. Moves almost exactly like a thin man. Has a flashbang and a smoke grenade. Cost equivalent: 1 thin man, 2 sectoids, or half a muton. You can buy 5 of them and have 3250 left over, which is enough for a mechtoid or a sectoid commander. Or a rocketman heavy. Or a MEC.