Defensive Offensive is your most critical tool. A ton of his attacks have small hit windows so a good D/O player can keep up a relentless offense on him and get him down quickly. Once you get him down to 150% you have completed most of the fight since the rest is mostly 'events' with maybe a bit of offense here and there if you keep up a strong offense.
Personally, playing without music (which kind of stinks) helped me get the timing for the invincibility frames of the D/O and it helps since, quite frankly, he has several attacks that are unblockable and the only chance you have is the to run or D/O. The grab is probably the most annoying unblockable since it comes out really fast and a second too early/slow means he has you. Best option is to do a directionless D/O to get some space.
Once he is 150% he will start tossing debris at you. Your best option is to slash but if it is too difficult run towards him to dodge it. I highly suggest you learn to cut the stuff since it does damage to him and earns you BP for the S rank. My advice. Use both thumb stick and make your right hand a hook. Use your thumb to angle a slash and your index to hit the X/Square to slash. If you line up all four boxes it will be easy since you have tons of time to do this (on revengence anyway).
Once he is down to 100-120% he begins to heal. Get behind him, blade mode and stop it. If you have Wig B you can go to town on him and get HUNDREDS of slashes on the guy essentially pushing him to his end phase if done correctly. If not you can push him to his 60% heal rather fast since he does another toss shortly there after. If you don't have Wig B then you can still do tons of damage doing a strong offense to push him into a throw/another heal. Finish him, you are a GOD! Take note a 100 slash/BP pickups must be done as at least ONE of these is necessary for an S. NO damage only, like Sam, will not give you an S. Because, as I quickly learned, this game f***ing hates you.
As far as his other 'fights'. The fist fight is easy peasy. Just dull. It is timed. Don't fight. Run like a pansy and after 2 minutes-ish He will flash gold and a cutscene will play.
Excelsius is another story. Again an easy boss with powered up pincers since pincers are hax on bosses. The only challenge is his slashes are 'unblockable' if you are going for no damage (and you are) then you must D/O the slashes which is easier than it sounds. After that you pincer his feet/zandatsu. This will stun him. Jack the Ripper/jump/ and do the twirl pincer move since this will damage him heavily, make sure to D/O or Blade Mode cancel to do even more damage. Next comes Gekkos. It sounds hard but really it is easy since he kills them for you. Run ALL the way back and dodge the lasers. The lasers will finish the Gekkos leaving them open to Zandetsus. There are 6-8 Gekkos in all and if you want an S then you must Zandetsu at least one of them. After that finish the last leg with pincers and you are done!