Ive been slowly working through the story, so i only cleared this myself a couple days ago. The healing potion you have is good for about 35k and usable every 30 or so seconds with no apparent limit. Any time you see your health dip into the 60s or lower, pop the potion. The boss has an unavoidable attack(sky-something) that does roughly 1/4 of your health - this is a good ability to use your defensive move on. I focused more on dodging then attacking when the dragon goes into seeker mode, using the jumping attack to get back into melee range when the last hit box drops. Pop your offensive buff as much as you can when you have a window to attack so you can shorten the fight a little. Then comes the big moves. When the dragon heads all target you, i found myself moving to the center of the arena before popping the invis move to give myself more room to maneuver into hitting him with the 2nd part of the attack. Everything kinda repeats, you do the big invis attack a second time. Then for the last part of the fight... lets spoiler this thing just to be polite...
When you try to do the invis move for the 3rd time, Ranj'it sees through the move and stops you - twice. At this point you need to weather his attacks for a bit while effectively super-paralyzed and only sometimes able to move. This is where keeping your health high for the rest of the fight will benefit you. Pop your defensive move, use the potion as much as you can, and if you can pull off any dodges do it. After a bit Thancred will say that he has one more trick up his sleeve, and a final invis move will succeed. From here you just need to set up the last hit and you win the fight.
Hope that helps a little. This fight is long enough that it might also benefit you to take a break and come at it fresh. Maybe level another class for a bit(do you craft?), then take another whack at it from a fresh perspective. Its certainly clearable(though a bit hard for sure), so best of luck!