Did you really do 400 duriel because that 800 of each mat for the summon, and the only way to pull that off is you or someone in your group buying Duplicated mats for Real money 
You mean like the gauntlet?
More or less yes 300-400 in not sure. For example i farm to do 4/5 duriels and with 4 players we can do 20 rounds.
You can trade mats with gold in game too.
I find other players in game without any problem.
For example last night we did 4x4 duriels and won other grandfather.
But im tired of farming for nothing.