As if normal mutons weren't bad enough, the aliens have developed even bigger and meaner versions of them. The berserker is equipped with a clawed gauntlet and will close the distance between it and your troops as fast as it can. When it feels like getting to your troops even if they're out of reach, it will bullrush straight at their cover smashing it's way through. Just to cause you more stress, every time the berserker is shot by one of your troops, it roars at them to try to intimidate them and then runs closer to them, even if it is your turn. Once you see one of these guys, have your troops focus their fire on taking it down before it causes havoc.
An update, as of Enemy Within, killing a Muton Berserker with a MEC's Kinetic Strike (melee attack) will activate a short fight sequence. As well as earn you an achievement for your first time this occurs.
As for Multiplayer, it's friggin' intense. My current loadout is a Sectoid Commander, Muton Berserker and a Rocketman Heavy (just a regular Squaddie Heavy, no perks) for a Rocket Launcher of course.
Sometimes I go three Chryssalids and two Sectoids (for scouting), but my first team can really destroy. Give it a shot if you're bored, because it can really be fun. Especially if you're up against a guy who has one relaly powerful unit, get a mind control on it and then destroy their team with their own unit. I did that to a guy with a super Assault guy and a little sectoid, and as soon as he tried to move the Sectoid away, the reaction shot from the Assault's perk killed it. My gosh it was hilarious.
Please watch me! It helps