Some things I have noticed while studying the way of the Smoldering Crab. (Applies to Melee only)
>I would advise keeping equip load under 70% while fighting them, you'll need the roll range. Potentially lose any shields too and two-hand your way through the fight. With enough practice and familiarity, becoming hit in the first place becomes extremely unlikely.
>They don't just hate strike damage, they REALLY hate strike damage. (I used a +4 Executioner's GS @ 38 STR, two handed) Bring a strike weapon with some reach.
>Their left claw, while faster, leaves them extremely open to at least two swings from a greatsword. Roll behind them, under the attack. 1st strike will hit their behind, 2nd will hit their front as they turn to face you. 3rd swing is possible, but risky. Their right claw will follow up its first attack with a second almost without fail, making rolling and countering this claw more undesirable as the left claw will not likely combo if you roll under it. The 1st swing after rolling can safely be an R2 attack if you land close enough and have a suitable moveset on it for your weapon.
>If the crab is only presenting its right, larger claw, roll under its attack, then roll immediately again away from but in front of the crab. This should stop it trying to smash you with the wrong claw and allow you to bait it into using its left instead.
Just my two cents, hope it helps someone new who is super intimidated by the crabs like I was when I started. Many thanks also to the other contributors here, this wiki has been such a huge help c: