Guys, for Power Construct I made this:

In 'PokeBattle_Battler' added:
  attr_accessor :captured
  attr_accessor :startform   # Greninja, Zygarde and Minior
  attr_accessor :originalhp  # Zygarde


In '  def pbInitPokemon(pkmn,pkmnIndex)':
    @iv[4]        = pkmn.iv[4]
    @iv[5]        = pkmn.iv[5]
    @startform    = @form # Minior
    @originalhp   = @hp   # Zygarde
  end

Inside '  def pbFaint(showMessage=true)':
    @pokemon.makeUnmega if self.isMega?
    @pokemon.makeUnprimal if self.isPrimal?
    if isConst?(self.species,PBSpecies,:GRENINJA);    self.form=0; end
    if isConst?(self.species,PBSpecies,:MIMIKYU);     self.form=0; end
    if isConst?(self.species,PBSpecies,:ZYGARDE) && self.form==2
      if self.startform>0; self.form=startform; else; self.form=rand(2); end
    end
    @fainted=true
    # reset choice

And works perfectly when Pokémon dies inside the battle.

To trigger the ability, I added below Moxie's code (inside PokeBattle_Battle script):
    for i in priority
      next if i.fainted?
      # Power Construct
      if i.hasWorkingAbility(:POWERCONSTRUCT) && isConst?(i.species,PBSpecies,:ZYGARDE)
        if i.hp<((i.totalhp/2).floor) && i.form!=2
          i.startform=i.form
          i.originalhp=i.hp
          i.form=2
          pbDisplay(_INTL("You sense the presence of many!"))
          pbCommonAnimation("PowerConstruct",i,nil)
          i.pbUpdate(true)
          scene.pbChangePokemon(i,i.pokemon)
          i.type1=getConst(PBTypes,:DRAGON); i.type2=getConst(PBTypes,:GROUND)
          pbCommonAnimation("PowerConstruct2",i,nil)
          pbDisplay(_INTL("{1} transformed into its Complete Forme!",i.pbThis))
          PBDebug.log("[Form changed] #{i.pbThis} changed to form #{i.form}")
        end
      end

But if I add here:
def pbAfterBattle(decision,canlose)
  for i in $Trainer.party
    (i.makeUnmega rescue nil); (i.makeUnprimal rescue nil)
    if isConst?(i.species,PBSpecies,:GRENINJA);  i.form=0; end
    if isConst?(i.species,PBSpecies,:MIMIKYU);   i.form=0; end
    if isConst?(i.species,PBSpecies,:ZYGARDE) && i.form==2
      if i.startform>0; i.form=startform
      else; i.form=rand(2)
      end
      if originalhp>i.totalhp; i.hp=i.totalhp
      else; i.hp=originalhp
      end
    end
  end
  if $PokemonGlobal.partner


After battle, give me error because the game didn't recognize 'startform' (possibly originalhp too). Here the issue:
---------------------------
Pokemon Essentials
---------------------------
[Pokémon Essentials version 17.2]

Exception: RuntimeError

Message: Script error within event 16 (coords 26,6), map 3 (Red's house):

Exception: NoMethodError

Message: PField_Battles:395:in `pbAfterBattle'undefined method `startform' for #<PokeBattle_Pokemon:0x9ae2ea8>

***Full script:

#pbDoubleWildBattle(:KAKUNA,95,:METAPOD,100)
pbWildBattle(:REGIGIGAS,55)


Interpreter:243:in `pbExecuteScript'

PField_Battles:390:in `each'

PField_Battles:390:in `pbAfterBattle'

PField_Battles:102:in `pbWildBattle'

PField_Battles:98:in `pbBattleAnimation'

PField_Battles:98:in `pbWildBattle'

(eval):2:in `pbExecuteScript'

Interpreter:1606:in `eval'

Interpreter:243:in `pbExecuteScript'

Interpreter:1606:in `command_355'



Interpreter:276:in `pbExecuteScript'

Interpreter:1606:in `command_355'

Interpreter:494:in `execute_command'

Interpreter:193:in `update'

Interpreter:106:in `loop'

Interpreter:198:in `update'

Scene_Map:163:in `update'

Scene_Map:161:in `loop'

Scene_Map:170:in `update'

Scene_Map:234:in `main'




What could be?