Always do harcast then a cast of anything instant after.
I.e aero 2 -> aero
Medica 2 -> regen on tank
Holy -> swiftcast holy if its ur last holy. As if you do the instant spell cast first, you'll have to deal with the GCD before you get to cast again. Anything you do will persist onwards even if the effect leaves you
I.e CS aero 2 & aero will still tick for CS dmg even if u turn it off after casting them.
Divine seal medica 2 and regen will still tick for more even after divine seal buff wears off.
Only spam holy if you have shroud up as it'll allow u to recover mp while losing mp, so the drop shouldn't be too big.