Hello,
For the most part without any skill points or defining skills like Cover or Full Swing tanks are just another DPS. Speaking specifically for the level 30 PvP, 9 times out of 10 you are just going to rush healer down then clean up. White mages have trouble healing without Regen and SCHs will get picked apart by sleep if your team has it and is coordinated enough to utilize it as they cannot remove conditions til 40. If you're trying to kill a target try to save your stuns until they're about 60% or lower and you have another dps helping you out. This is usually when you apply Full Swing if you have it since you're trying to burst your target for a kill.

DO keep an eye out for your healer though as you can easily be their best friend. If you purchase Testudo, make sure to use it as soon as you get spooked as it's worthless if you lose your healer. (1/2)