I tried playing the endless mode for about fifteen seconds before I gave up due to the auto facing an enemy near you feature, and moved on to going through easy mode getting my collectables and weapon related achievements.
By the way, within those fifteen seconds I did get two weapon related achievements, though. From what I could see, those levels have a lot of unique weapons in them, and a lot of enemies, so they'd probably be good spots to grind out the weapons, if you wanted to.
LOL to the above.
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