If you want instacast Severe Rainstorm then 120 DEX + temporal dex boots for footgears slot is a must, then 2 Sound Amp for accessory slots to get rid of the variable cast time. Weapon for it is ofc, highly upgraded Elven Bow + Elven Arrows, for Armor, I use a Bryn, but any Porcellio carded amor is fine, +9 FAWs EA10 is suffice, but if you are on a budget then a +9 HBP should do it.
As for top headgears, if you want to maximize your dmg, then even a +0 Lyrica hat is enough ( the upgraded bonus sucks) I carded mine with Dolomedes card and upgraded it to +9 for 5 extra DEX, but Dark Pinguicula is a good choice too.If you want to reduce on consumables use then you'll need a +9 RWH or Vanargand , RWH for HP leech and Vanargand for SP, and finally, BDM or Dark Pinguicula carded slotted sunnies/BFG... for mid headgear.