Trying to solo him?
How much evasion/evasion skill do you have? Merits?
If you want my advice, go /WAR and bring a set of tools to spin the wheel. You want the attack to over-come his DEF and the wheel dominates when he goes into his physical damage reduction mode.
I have capped merits for anything ninja related (5/5STR, 8/8ktn, 4/4eva, 4/4crit, 4/4spell interrupt, 8/8ninjutsu, doton, san spells etc.) As well as a 299 eva skill setup for the fights and a full eva set.
I'd go NIN/WAR but I always seem to get hit at least 2 times before I time out/get borked and NIN/DNC has yielded far better results for me so far.
Biggest issue I have is that I'm usually too cheap to spring for elemental tools, and every time I've tried to use them, he gets a lucky double/triple attack and I get boned.