Sarth is simple enough, i know of at least one boxer that has done him with 5 characters.
The real problem there is the drake that goes immune while the mob inside the portal is up, requiring a second tank/healer or so to go into the portal to kill it.
I have plans for 5-boxing it myself, but so far that is what is stopping me from clearing OS10 by myself.
I'll prolly do/try some more bosses in naxx10 this week, I have some plans for how I'll do them ^^
As you can see in my video I did it with one tank and one healer. :)