Just beat Celestial Emissary. I almost got him first try - one more hit would've done it, but I ran out of stamina at just the wrong time and he splattered me with celestial spooge. I'm beginning to think Blades of Mercy aren't as awesome-sauce as they first appear. All the aliens appear to be resistant to slash, and BoM only do thrust damage on fancy roll/backstep/etc attacks (same ones that proc the arcane damage, it appears). So I used some of the billions of shards and twin shards and a few of my oh so precious chunks to get my rapier-gun thing up to +7. Unlike BoM, the rapier can be buffed, so a dose of bolt paper and stabbing him in his celestial goolies dropped him super-quick. I have a suspicion the BoM is more of a PvP weapon.
The rapier-gun R1 alternates between thrust and slash damage - you can tell just by looking at the moveset - and when using it on aliens the first attack (a poke) does lots of damage, the second (a swing) does only a little.