Ah yes, the Thancred fight against Ran'jit. That has a specific strategy to win in that you need to use Thancred's abilities effectively - specifically his Hide ability. Because Ran'jit just hits far too hard, simply going in and trying to tank and spank him normally is tantamount to suicide. You basically have to quickly rush in, hit Ran'jit, run away like hell and then hide while trying your best to avoid Ran'jit's attacks (as well as being mindful of the toll it takes on Thancred), and then rinse and repeat.
Then, towards the end when the option becomes available, use Ryne's 'special' cartridge (that is on the Duty Action button) to execute an extremely powerful attack that practically one-shots Ran'jit and then completes the battle. There are some other things to remember but I can't recall them from when I did that annoying fight.