A player gets in front of a boss lair and two or three other players jump in. As a result the boss has 3 minions instead of the standard 1. But when the player moves forward to attack the other three players don't move. After the battle begins they move forward but by then it is too late. All the monsters have targetted the original player. The three players then proceed to cast nothing but defensive and offensive-enhancing spells until the original player has been defeated. Then they finally unleash a wave of high power attack spells and cruise to an easy victory.
This happened to me twice where it was obvious I was being set up as the "fall guy". Afterwards I got a little paranoid. Sometimes I'd just stand there. "I'm not moving an inch until you do". Other times I'd just back out and start a new instance. " I don't know you and I don't trust you."
There are two ways I can see of stopping this from happening.
You could set it up that once one player engages the enemy, every other character who entered the boss lair is dragged into the battle whether they moved or not.
You could have the boss always start with the standard number of minions then add a new one every time another player enters the battle ( Targetting the new player of course ) similar to what happens in street battles. This would have the added benefit of making boss battles harder for those who teleport in.