here's your basic framework for what you'd want for the ultimate tanky Crab bushi:
You'll want to consider spending xp at creation to get either heavy armor made in the Water Hammer city for +2 reduction, or Heavy armor made by a rank 5 Kaiu Engineer, who will give the armor +5 reduction and +3 TN if I recall.
you'll also want to get the extra tattoo advantage (crab tattoo) for reduction equal to your earth
once you get to rank 2 crab bushi, you'll probably want rank 4 earth, since you get reduction equal to your earth at rank 2
and then you'll want to join the Defender of the Wall advanced school
the requirements for it are: 4 earth, 5 strength, 4 defense, 4 heavy weapons, 5 lore: shadowlands.
the 2nd rank of the school is where you get reduction 8 (so you're total insight rank 4)
also, when considering kata, look at Strength In Arms Style. 4 water mastery. Once per Turn while wielding a Heavy Weapon, you may use your Strength Trait instead of your Agility for an attack roll.
since you're skipping rank 3 crab bushi, you don't get a 2nd attack per round for a verrry long time, and you need 5 strength to get into the advanced school, so you could just ignore the fire ring for the most part, and you aren't sacrificing your attack bonus
(insight rank 6 is when you could graduate from the advanced school and rejoin the Crab Bushi at rank 3)
and you're 2 kiho options that are strong contenders are Embrace The Stone 5 earth, By focusing their chi like that of the mountain, monks may become like stone themselves, turning aside blows and ignoring damage. When you activate this Kiho, you gain a Reduction Rating equal to twice your Earth Ring. This stacks with any other Reduction you have from other sources (such as spells or armor). This effect lasts for a number of rounds equal to your Earth Ring.
and Ride The Water Dragon 3 water To walk with the Water Dragon's power is to harness the incredible restorative abilities of the ocean. While this Kiho is active, for a number of Rounds equal to your Insight Rank, you recover Wounds equal to your Water Ring during the Reactions Stage of the Round.
the earth one is bonkers when you stack it on top of everything else, but then you are reliant on shugenja to heal you. or, you could take the water one and start self healing right away, and consider stuff like magic resistance
since you need so much strength to qualify for the advanced school, you should also consider raising perception up. I don't remember if you need to have water 4 to learn a 4 water kata like the one i mentioned. I think so? getting your perception up will be good to increase your insight efficiently. so you may also want perception based skills like investigation (like you needed even more reason to take investigation lol).
and with that much lore skill, you might want to consider getting intelligence up to a 3 so you can actually roll ok on that lore skill
rolling 7k2 is pretty bad, considering you have 5 ranks of the skill
so, adding up the reductions: 5 heavy armor, 4 earth rank 2 crab bushi, 4 earth crab tattoo, 2 kata, 8 rank 2 defender of the wall. this is 27 total. and you still have the option to get a bit more from specially made armor. and you haven't picked your kiho yet either.
and if you get earth to 5, you get 29 reduction, with the option of 10 more from the kiho
the interesting thing about this build is, since you have so many hp and so much reduction, you won't actually need to spend void on reducing damage very often.
oh, right, and if you take the healing kiho, you need 3 water to qualify for it, so you probably need 3 water early on anyway
that's all I can think of to max out the reduction. you're kind of locked into being a stereotypical beefy crab wielding a heavy weapon, but if you want the tanking sonuvabitch that ever lived, this is it
and it only gets tankier the more earth you gain
and, something i literally just thought about, is once you graduate from the advanced school, instead of going back into the crab bushi school, you could become a monk. by that time you would have 5 earth for sure, and could get some more kiho from being a brotherhood monk at that point :)
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I'm going early water tattoo I think, get that early healing in.
Thank you Tadaka! Sent to me 2022, feel free to make your own :D