Crit is the top priority, followed by C.A. DMG and C.A. DMG Cap. The rest of your choices are pretty free, but since Threo works best in Enmity grids, consider placing points in DEF and HP.
DA and Crit are staples of any EMP build and should be prioritized first. Her EMP passive is especially useful over long fights, providing an indefinite 30% stackable ATK boost, so long as it doesn't get dispelled. As Threo will be using a lot of charge attacks, raising C.A. DMG Cap will allow her to deal more damage every time they're used. If the cap is not being hit consistently, then points can be invested in C.A. DMG instead.
The last 3 points are flexible. Typically putting them in HP is a good investment until she has about 9,000 Natural HP, or 12,000 Natural HP after her final uncap, in order to boost her Ground Zero damage. After these thresholds are met, the stars can be invested back into CA DMG and CA DMG cap up. At least 1 star should be put into each of the CA DMG nodes in this way. The ATK Up/DEF Down do not provide enough stats to justify her decreased survivability. She could take nodes in Defense to offset them, but they're better off used in increasing her charge attack paramaters. ATK and Water DMG down scale poorly, and should not be considered.
For her Over Mastery, Threo can benefit well from whatever 3rd bonus is given to her. Priorities should be given to Enmity and Charge attack parameters, while Stamina and Debuff Success should be avoided.
On her fourth bonus, she will want to have either boost to Multi-attack, although she can work with most options provided she has a good boost to Enmity or Charge attack Specs attached to it.
