Anyway the great advantage of multiboxing IMHO (but I know that some people here don't agree) is that you can pick whatever class you want, and then combine offensive or defensive bonus. And this is where 4x Invoker Crusaders are bad. They are just 4x the same DPS dude... What I did is run 3 different laws, and the 4th guy uses Condemn/Vaccum to gather critters, but that's a very minimal combination.

Obviously to break that wall we need to have one character replaced by another class that buffs at least 33% damage (to maintain the same overal damage), and gives extra protection. I could think of a Monk here. Or eventually a WD, however a WD offers a huge DPS buff and no direct protection (but he can incapacitate critters a lot).

I'll try various combinations during the coming weeks, and see what works.