Using Stoneskin I as the last cast before the mob dies is a good habbit. Try to do that.

Personally I use Swiftcasted Stoneskin II only at the start of the dungeon. It is much better to save it for Aero III or Holy. Sometimes I use Stoneskin II when there is offtime standing, but usually it is either Stoneskin I or when the party is not in danger of big aoes Medica II to fix the things. Don't be afraid to over heal a bit, the tank should be able to take it, and if the team is damaged you also get more time for dpsing.

Keep in mind that Aero is 200 potency and Stone III is 210 potency. It costs less MP, but unless you are moving it is dps loss.

btw. You can edit your post to cheat the character limit. For some reason this small limit hasn't been adjusted as of yet.