I did both of them at lvl24 and they were hard. You just need to be cautious and use range to your advantage. I have a really nice sniper rifle that kills most things with one or two headshots and then a legendary rocket launcher for some mayhem. Like atpbx, I managed to kill the knell on his barge from the doorway using a rocket launcher with 2 in a magazine. They were both in the air before the first one hit.
In general the hardest bit of them is getting to the last boss.
That being said, I have been rewarded with 2 bits of armour that I don't need. It has allowed me to deconstruct them for parts to upgrade my chest plate though.