This boss isn’t as tough as I’d originally thought. I found a decent tactic to make it a little more trivial. I did this on NG+ with a +5 quality long sword. I’d also recommend using a flame resistance ring and a regeneration ring to counteract the fire damage you’ll take from the boss and parts of the arena.
Your natural reaction to Flamelurker will more than likely be to find a 100% damage reduction shield and just block and wait for your attack/heal windows. The issue here is that you’ll take a bit of chip damage and your stamina will be wiped out by most of his attacks, leaving you wide open. Flamelurker hits hard enough to almost one shot you in these cases.
I found the best thing to do is double hand your weapon, use free cam and just sprint around the arena keeping track of him holding your controller with the ‘claw’ grip. This way you’ll maintain enough distance when he attacks whilst also having time to go in for a couple of hits during his recovery animation. Go in for a couple of hits but leave yourself enough stamina to create space between you and him before he attacks again. Try not to get carried away if you see him stagger when you attack as he will be able to recover and get his blows in on you fairly quickly. Just rinse and repeat this process. As the fight progresses he’ll get more aggressive in his pursuit of you. Just keep running away, manage your stamina and be smart with your attack/healing opportunities and you’ll finish him no problem