Ah yes, this one was... far from fun. I mained BLM through 4.0, only switching to RDM before doing 4.1, so I did this one as a BLM, and boy was it infuriating at someone who plays with a smaller screen so I don't lag as much. In case you haven't beaten this yet, or plan to try again with BLM, the key here is making sure none of the assisting Knights die. That means when "Inquisitor Guillaime" gets to 90%ish and spawns those wyrmhounds, you need to immediately target that one up on the ridge and aggro it. The video I watched to make sure which fight this was, they wyrmhounds came down, but in my variations they didn't, so I had to run up with the knight, scathe the wyrmhound, and bring it down so I could slaughter it, otherwise it would slaughter the archer, and you definitely need them later, since you don't get to stand still much for this fight. Sleep whichever ones you're not downing, take them all down quickly, then go back to the boss from there. The biggest thing is to make sure that your NPC friends aren't taking much damage in the first phase before he transforms, because they are your saving grace afterwards. Staying near enough to get healed while laying DoT and whatever decent spells you can around scathe are your best bet. It's an absolutely hellish fight for a BLM with all the movement, and moreso because you need the NPCs to help you out, but you'll want to get used to it. The BLM job quests are as equally hair-pulling later, to test your worth.