Stoneskin

[YanDere]

If I had infinite mana I would recast stoneskin on the tank anytime everyone's HP are full. But as I'm learning battles I'm afraid to waste the MP.

For example, I'll be doing Titan (level 34) for the first time, and know it's valuable to cast a certain times, but afraid to waste mana going for maximum uptime :/

You don't need a particular strategy in Normal Mode, just avoid aoes and stand as far as you can from the middle during Geocrush.
Stoneskin or not, nothing changes.
I find really useful to cast Stoneskin on the tank before Mountain Buster in Titan Hard Mode, and I usually try to swiftcast Stoneskin on the tank each pull, but only when we are doing 10 min speedruns.
Until lv50 you don't need to worry that much about other than healing skills.