PLD is a tough one because you have to be hugely aware of what is happening and how other classes work. If people tell you to be on Healers/Casters 100%, ignore them. You can't stop that healer from casting once they are resist, you can't stop a BLM from knocking out Fire IVs if they are resist too.

It's about picking a good time to stun, BRD popped Barrage? Stun lock him instantly and finish with a Glory Slash. MCH got all buffs up and popped Rapid Fire? Same again and wave goodbye to MCH for a bit. Same again for when a BLM got leylines and Enochan up; when the healers are having a hard time with enemy melee, stun to throw them off; if a enemy DPS is nearly dying, do it to a healer. You aren't supposed to be on someone 100% of the time, you do nothing to your group if you do this.

Testudo should be used fairly short into the game and then used pretty much on CD. For tank LB, you need to be aware of enemy LBs. If you just got your LB, 90% of the time an enemy DPS has just got their LB. Read your enemies and judge if you think they are about to LB and use yours.