I have a quick fix for the deathsinger twins for the raid. As everyone has come to notice, once you kill the deathsingers in the raid, you do not get their grimoire card. Here is an alternate method to get them. This method takes a bit of time and uses a LOT of special (bring plenty of synths). Do the mission where you have to steal the essence of Crota's soul. Once you get to the end, pull out your sniper and CAREFULLY, shoot ONCE at the deathsinger. (You will be in third person mode, so aim carefully) You will turn red, but do not panic, as you will fade back to normal. Continue this process until you have killed the deathsingers. Please note: DO NOT LEAVE THE POOL OF WATER. If you do, you will be spotted by the enemies that are "patrolling" on the floor above you and you will have to start over. If you run out of special and do not want to wait for your synth, you can crouch in the very back of the room and waste your ammo for charity ammo. Rinse and repeat until dead. Yes you can use rockets and other weapons, just be careful not to shoot more than once and do not use rockets that have cluster bombs as they will damage other enemies and you will be spotted.
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Ice Breaker works very well for this!