I didn't do any Blackram/Bamboo PVPing.

I didn't do any Blackram/Bamboo PVPing. 

As you can see here, those are Cheap Blade And Soul Gold a bit different achievements; "Opposing Faction" means killing the opponent from an opposite (Cerulean/Crimson) faction, no matter what they're wearing. 81 includes Arena PVP kills, not involving any faction uniforms.

Ok, I wasn't aware of this. If I understood this right; if two players belong to the same faction but kill each other in opposite AGs, your achievement would not increase, right? It leads me to assume that one of the best way to raise this is using Arena.

15 is the number of players wearing Crimson Legion faction uniform killed in op

This is what I wanted to know. This means open world pvp, while wearing opposite faction uniforms counts. You haven't done Blackram/Bamboo pvping, I wanted to know if the same achievement "Double Agent" would increase during Blackram/Bamboo pvping. If this was the case, then I would be able to raise this achievement without siding with a faction. How else can one kill both factions at the same time and have it count towards this achievement?

Ok, I wasn't aware of this. If I understood this right; if two players belong to the same faction but kill each other in opposite AGs, your achievement would not increase, right? It leads me to assume that one of the best way to raise this is using Arena.

This is what I wanted to know. This means open world pvp, while wearing opposite faction uniforms counts. You haven't done Blackram/Bamboo pvping, I wanted to know if the same achievement "Double Agent" would increase during Blackram/Bamboo pvping. If this was the case, then I would be able to raise this achievement without siding with a faction. How else can one kill both factions at the same time and have it count towards this achievement?

You can see that Double Agent actually requires you to kill Cerulean and Crimson members, and only counts within open world PVP wearing the PVP outfit. You can't complete it without switching faction.