I know that Avesta has done this before, but I decided a few nights ago that I was going to give it a shot as well as I'm always looking for something new to try. After killing Zip with a ninja friend of mine and trading a Ro'Maeve Water to a LS in sky for the Winterstone I was set to go. I grabbed an Ambrosia, bought 10 stacks of Antidotes, switched to /NIN, and headed up to sky prepared for a long siege of the turtle.
Let me just say that the fight itself isn't overly difficult, it's all about pathing and patience for the most part. Your main damage by far is quite literally just Bio II, as his magic resistance for a RDM is almost ridiculous (tier 3 spells hitting for like 28 damage all the time) and the fight feels like you're trying to take out a wall with a box of toothpicks. After roughly 3 hours of battle though his life was finally down to just a sliver and I took him out with a rather buff Thunder III for 230 damage.
My rewards for all this fighting was an aquarian abj hands and an adamantoise shell, meaning that I literally lost money on the fight compared to the antidotes I had to buy, but in the end I wasn't really fighting for money at all, it was more of a test of skill and patience.
Now I present to you, the Official Prima Guide to Defeating Genbu Solo v1.1
1) Always keep 3 shadows up, if he ever attacks or a spell hits you just recast Utsusemi. This is so that you don't get unlucky and have him double attack > turtle stomp you (never should happen if you stay away but safety is first).
2) Always keep Bio II on him as it is your main damage. He's basically immune to poison and other DOTs aren't really worth the mp, so make sure your one DOT that works is always active.
3) After he casts Poisonga III, immediately recast utsusmi and antidote (preferably macroed) so that you don't have to waste MP curing yourself.
4) Kite back and forth on the island, turning around at two spots: You don't want to be 180ing at a point where he is going to be able to hit you, so my 2 recommended spots would be the trees with the opening between them right before the area leading to the teleporter off, and the other is the platform right after the platform where you enter his island. When kiting around the trees it's best to pull him around the right side and then run left and cut between the trees. His pathing is a little quirky here, sometimes he'll go way around and sometimes he'll come at you, and this is the only time I ever got swung at (and only a couple times total).
The other 180 area is even easier because his pathing is completely terrible and he never even tries to come at you. After you cut back through the middle of the trees and keep running you'll go down a little ramp towards the teleporter you came from, that is where you'll turn him...don't go all the way to the teleporter though as the area is too small. You'll quickly notice that when Genbu comes down the ramp he always, always just keep running straight for some stupid reason even if you're standing 3 feet away from him to his side so he should never even touch you.
5) Now you have your damage and your kiting, so you can try to make the fight more efficient a couple ways. First of all, since you have convert every 10 minuters you might as well use your MP nuking, so whenever he starts casting Water IV or Flood, stop kiting and run back and nuke (Thunder III did the most for me), for Flood you can get Bliz and Thunder both in. I'd leave about 150 mp always as a precaution but burn the rest if you can.
6) Another way to conserve your MP is to never get hit with Waterga III, and the best for this is simply (and obviously) always running away. The trick here is getting him to cast before you get to a 180 so that he doesn't cast it while you're trying to run past him. If you notice with magic casting mobs they have a timer that counts down and when it's at zero they will cast. Well, if you're always too far for him to use magic his timer will sit at zero and then when you try to 180 and step into range he will always use magic and you'll get hit with Waterga. What you want to do is to stay within casting range before the corner so that he'll attempt to cast before you get to the 180 and you can run out of range of the spell, and then you can safely 180 without fear of him casting during it because his timer isn't ready.
7) Beware his savage 2 hour (invincible) and Harden Shell (protect) and Turtle Song (removes bard songs) spamming, as they are all extremely dangerous moves that will quickly turn the battle in his favor.
There you have it, now go take Genbu out yourself it's not really that difficult at all when you give it a shot. Keep in mind that if you're planning on going to the bathroom or if you had Taco Bell recently that the fight might be better off postponed until a later date as it will take a long time. Also be sure to put an apology in your search comment for all the LS waiting to fight Genbu that night, as the island will be shut off for hours and they'll sit there confused as to what is going on.