What game mode are you talking about? 

Play a void crusade on hardcore mode, it is solo as the void crusades don't allow co-op, but they are single player modes. Also, once you hit 10 to 11 levels higher then your current one, you are doing -99% damage base whilst the enemies keep stacking beyond 500% damage whilst significantly upgrading their health, so that becomes quite a challenge to survive including once you hit the high 90's and above and have endgame gear set ups, but I'm, curious to know which infinite spawn play mode you are talking about.

There is the unholy cathedral with infinite spawns of demons, and 2 other ones that I dont bother playing.. Valorous Heart and something else about revenge or something.. basically fighting infinite waves of enemies.

Valorous Heart and the Drukari one with teh soul harvesters are actually the most efficient way to farm purples and oranges. You of course need to refine a build that kills most of the trash mobs ion one hit and the champs in a few only and then yoiu need speed tro push into the 6,000+ vengeance levels where you are raking in 12+ caches a mission, it's not easy to get to that point mind and it does take some strategic thinking, but it's doable. If you are looking at refining builds you need parts and the excess drops you won't use out of these missions' rewards are a very good way of getting those resources. 

I was helping friends with 10 missions on eye of terror and only the first 3 missions had infinite spawns, then the data collect mission came and after the 3rd death respawn was greyed out and once we both died 3 times it was failure. Almost every mission after that has been the same. Kind of makes the perk for crusader to take less damage that stacks indefinitely useless. So I don't know which missions you are getting this.