Stoneskin + Mercy Stroke

[Shuon]

Stoneskin is probably our best CC skill, but very situational, and you probably won't find much use for it until HM Primals and onward.

Absolutely not true.

In Haukke Manor, I always had time to apply SS prior to Void Thunder III. In fact, I always had time to re-apply SS whenever I fought bosses. It helped with healing, a lot.

In Brayflox, Aiatar takes really long to charge up it's dragon breathe attacks, and I would use it to apply SS. It would then last for 1 to 2 hits, which was significant, and it happened really frequently.

In Temple of Qarn, I had soooo many opportunities applying SS fighting the Temple Guardian, although Adjudicator did involve some movements to make casting easy.

In Cutter's Cry, I could apply SS when the worm boss started burrowing, and when it randomly attacked other members. Chimera also had a lot of 'openings' for me to apply SS when it was casting it's various forms of attacks. SS was VERY useful in CUtter's Cry.

In Stone Vigil, once you have hidden behind the first boss when it's doing its lion's roar/breathe, it was huge window to apply SS. Isgebind also gave a lot of windows to apply SS when it flies up, and even before the frost breathe attack, which helped to mitigate a lot of damage since it hit pretty hard.

In Darkhold, the final boss doesn't attack in quick succession and there are also ample opportunities to apply SS.

[Stryker]

I grinded Con up to 34 for the skill before I lvled my PLD. It's a great skill to have even if you lvl up other classes. I used it heavily in my pre 50 dungeons since it gave me a couple of free hits. Especially if I didn't know what all the abilities were of the bosses I was fighting. Doesn't take too long to get to 34 with fate grinding, so its definitely worth the investment.

As for mercy stroke, I don't have that one. But as many other people have already brought up its downside, personally I probably won't go for that one unless I'm really bored.